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Here are some photos from visits to a Baltimore public school. Nuala and I shared science demos with every student, one class at a time. Thanks to Tani for sharing the pictures!

See the pictures →

1. Make a fist around two fingers (borrowed from someone else’s hand) and squeeze hard for half a minute or more. Then relax your fist, but don’t open it. Delicately slip the fingers out, and then gently try to open your fist. What does it feel like?
2. Stand in a doorway. Push your arms outward, pressing the backs of your hands into the doorframe, hard. Do this for at least half a minute. Then step out of the door, relaxing your arms and shoulders. What happens?
3. Lie on the floor on your back. With your legs straight (knees locked!) have a friend raise one leg and press gently so the leg muscle is stretched. It should be a little uncomfortable (and definitely a little weird) but not painful. Pause for at least half a minute. Close your eyes while your friend lowers your foot slowly back to the ground, holding by the toe. What does it feel like?

The first one in particular is good for moments when you are waiting around or standing in line with a friend. If you know more of these, post them in the comments!

Carbon dioxide (CO2) is heavier than most other molecules and elements in the air.  In the same way that Helium rises (think of party balloons), CO2 sinks.

With a very large bowl or container and lots and lots of vinegar and baking soda, you can make a little “puddle” of CO2.  When soap bubbles – which are filled with normal air – hover over this puddle, they float in place like beach balls sitting on top of a pool.  You can see this in the picture, but of course it’s much weirder-looking in person.  Bubbles serenely unmoving in midair.

A Giant Bowl. You need a bowl big enough that the CO2 can slosh around without completely seeping away.

Recipe for CO2. A back-of-the-napkin calculation shows that one big box of Baking Soda is good for about six big bottles of vinegar.  Their reaction produces CO2 gas and a big mess.

Bubbles! The classic recipe is 12 parts water to 1 part blue Dawn dish detergent.  A few tablespoons of glycerin (which can be bought at a pharmacy) help the bubbles last longer.  We tried some variations, like all-glycerin bubbles.  It also seemed like a good occasion for the Bubble Thing!

Click an image to flip through a larger view.

Finding the CO2 line. This Four Gas Tester, designed to check if air in a workplace is safe to breathe, showed that the air was normal above the bowl.  But when I dipped the sensor under where the bubbles were floating, red alarm lights indicated that the air was not breathable – too much CO2!

1. Get a medium-sized plastic soda bottle and a party balloon.

2. Puncture a hole in the side of the bottle near the bottom.  It’s easy if you use a nail heated with a match or a candle.

3. Dangle the balloon inside the bottle and open it over the bottle’s lip.  (See photo.)

4. Inflate the balloon inside the bottle.

5. With your mouth still on the bottle, place a finger snugly over the hole in the side.

6. The balloon stays open on it’s own!  Fill it with water from a faucet.

7. Bring it outside, and take your finger off of the hole…

Science! To quote Zap Science (see section “Soda Bottle Science”), we live at the bottom of an ocean of air.  Air pushes in upon everything.  The weight of the atmosphere pushes down and outward on the balloon but, when the hole is covered, there is not much air pushing back.  The pressure of the atmosphere – all the air piled up over your head – is working to hold the balloon open.  When you uncover the hole, you let the atmosphere back in (so to speak) and – splash! – the balloon is free to shrink again.

Stray tips. It’s best if you use a normal rubber party balloon, not a small water balloon.  Also, 1-liter bottles are best.  2-liters tend to fold in on themselves to easily under pressure.

This takes some prep work, but it is downright stunning.  A small spoonful of innocent-looking white powder is stirred into a glass of water.  A few moments later, when the glass is turned up side down, the water is weirdly solid.  You can even shake the glass up side down!

This powder, “super-absorbent polymer” is used in disposable diapers to help prevent leaking.  Each little grain of powder swells into a gelatinous bead that traps water.  (See the last photo.)

Where can I find this stuff? The easiest way to get it is to tear apart an (unused!) diaper or two over a cookie tray.  Really rip apart the cottony insides of the diaper, releasing the white flecks of powder that are dispersed through it.  Tip the tray to shuffle the powder into one corner, and collect it.  Discard the rest of the diaper.

The Second Act. You’ve seen the first part: simply stir a spoonful of powder into a glass of water.  The water will be “solid” within a minute or two.  But there’s more!  Add some sidewalk salt (active ingredient: calcium chloride) to the glass and mix it in.  The salt effectively disables the polymer, and water comes back out.  Try it with table salt too (sodium chloride). What happens?

Safety Note. The super-absorbent polymer is safe – it’s in diapers, remember – but if you’re collecting lots if it, avoid breathing its dust.  Sidewalk salt, while not too serious, is less people-friendly.  Safety glasses and dishwashing gloves are recommended.  Wash the glass thoroughly after this demo.  If you do the part with the sidewalk salt, use a plastic cup that you can throw away.

I’m usually interested in science that is easily shared – weird things to notice about everyday objects or materials. But here I will share two rare glass objects that I use in science demos.

Glowing Glass. These glass squares look plain and unremarkable. Under Ultra Violet light, they become brilliantly colored: red, orange, yellow, green, blue, and purple. Each square is doped with a different chemical that fluoresces under UV light, producing a vibrant rainbow.  They were made by an expert in this sort of chemical trickery, Dr. Matthew Dejneka.

The flashlight lighting the glass is also pretty cool technology: it has four UV LEDs. To our eyes, it looks dim and vaguely purple, but it is extremely bright in the (invisible) UV range.

Weird Magnification. A usual magnifying glass bends the light reflecting from an object to create a larger image. This “taper optic” works in a totally different way. Notice how it is different from a magnifying glass:

• A magnifying glass distorts the image, especially around the edges. The taper optic’s image is flat.
• To read a magnifying glass, you have to move it around to get the words into focus. The taper optic sits flat on the page, and the words look as if they are written in large print directly on the glass.
• A magnifying glass focuses a bright spot at the center of its view and it darkens the rest. The taper optic brightens everything evenly.

What is it? It is bundle of tiny glass fibers, fused together into a solid chunk of glass. The fibers are thin toward the bottom and thicker toward the top. Each fiber carries a piece of the image and spreads it out on the top surface, enlarging the print. (It’s a two-way street: Fibers also carry light down, concentrating all the light that falls onto the top surface into a small area on the page, making the page a little brighter.)

Why haven’t these replaced magnifying glasses? They are too expensive. A good-sized one is in the $80 range, and as of this writing they are not manufactured by anyone. I got mine by emailing the inventor, Harvard Professor Eli Peli. Mine is slightly marred, but it came free for educational use!

What happens if you flip it up side down? This.

Sharing the science at the Corning Museum of Glass.

As I bring several posts about science tricks online today, I’ll recommend these really great books by John Cassidy and Klutz Press.  They are activity books (they come with stuff attached) and read more like magazines then science books.  I’ve read mine countless times, and I’m always spotting clever stuff I haven’t appreciated before.
Explorabook
Earthsearch
Zap Science (out of print, but available)
Disaster Science (out of print, but available)

Also, I recommend books by Martin Gardner, particularly Entertaining Science Experiments with Everyday Objects.  The books are a little dated, but they have ideas you won’t find anywhere else.

Inspired by the well known YouTube sensation, I tried some variations of my own.  The co-creators of this famous Diet Coke + Mentos video, Fritz Grobe and Stephen Voltz, share their basic procedure in this PDF.  Obviously this is a pretty simple idea, but it is surprisingly tricky to get results as impressive as theirs, so reading their notes is worthwhile.

They are secretive about their more elaborate geysers.  Here are my ideas.

Magnetic Trigger. Grobe and Voltz suggest hanging the mentos inside the bottle on a short string held by a paper clamp.  For a quicker and more elegant trigger, use small strong magnets.  (One goes on the string inside the cap; one sits on top of the cap until it is removed, letting the Mentos drop into the Diet Coke.)  With a couple cheap electromagnets – even ones homemade from a nail coiled in wire – you could trigger multiple eruptions in perfect unison at the flip of a switch.

Whirling Bottle. In immitation of one of their geysers, I drilled holes in the side of the bottle above the liquid.  I suspended in the bottle from a wound string, and let it spin.  I want the eruptions to come from the sides, not the top, so I used the magnetic trigger.  (That way, you don’t need a hole in the cap.)

Science! Grobe and Voltz link to two good explanations.  The recipies for Mentos and Diet Coke are, of course, secret.  Exactly what happens is an unsettled question.  It seems to be a physical interaction having to do with the surface of mentos and the carbonation in Diet Coke, not a chemical reaction.

Other Results?

xkcd.com. Used with permission, under the Creative Commons Licence.

No explosives here, just spring-loaded good times.  This is in the spirit of (but not taken from!) The Dangerous Book for Boys: something everyone should try.

Get four popsicle sticks, preferrably after eating four popsicles.  (Use normal thin ones, not thick ones from a craft store.)  Assemble them as shown in the photos.  Weaving in the last piece is just a little tricky.  You’ll have to bend the sticks a bit.

If you need a closer look, click on an image.

Notice, in the final picture, that the sticks at the top are barely overlapping.  This is your “popsicle stick bomb.”  Throw it!

Made in a large supply, these are great for rubber-band-gun target practice.  Made carefully, they can be “chained” together.  …This is how I spent my childhood.

Science! If you want to slap an “educational” label on this most excellent of pasttimes, consider it a lesson in potential energy (bending the sticks) converting into kinetic energy (boom!).

Advanced Popsicle Stickmanship. A leaner shape makes a more powerful popsicle stick bomb. That is, squeezing the whole shape in from side to side bends the sticks farther and makes the “explosion” better.

Vinegar + Baking Soda is one of the staple science tricks of Kitchen Chemistry books.  Here are some fun variations.

1. Water + Baking Powder.  Not as foamy, but a little safer for small kids (no eye-stinging vinegar).

2. Vinegar + Baking Soda + Dish Soap.  The soap traps all the CO2 gas, and you get much more foam.

For more ideas in the spirit of Vinegar + Baking Soda, see Diet Coke + Mentos Variations.  For a more refined Vinegar + Baking Soda science demo, see Hovering Bubbles.