A boiled egg may seem unassuming, but under the right conditions, it can be a powerful thing. In this classic demonstration of how a vacuum must be filled, the egg withstands a mighty force of nature (at least for a little while), until the experiment's surprising -- and kid-pleasing! -- conclusion.
1. Gather a peeled hard-boiled egg and a thick glass bottle (we used a quart-size milk bottle).
2. Fold an 8- by 1-inch paper strip in half. Light it on fire (an adult's job) and drop it into the bottle.
3. Set the egg on top of the bottle. The flaming paper will burn itself out. Watch as the egg squeezes, slowly at first, through the opening and finally plops to the bottom.
The fire heats the air and causes it to expand. When the flame dies down, the air molecules cool and move closer together, creating extra space, or what scientists call a partial vacuum. The air outside the bottle tries to flow in to fill that extra space, but it's blocked by the egg. Eventually, air molecules outside the bottle exert so much pressure that they shove the egg in and rush in after.
To remove the egg, hold the bottle upside down and blow in the opening (a brave adult's job). Quickly turn the bottle away from your face and the egg should fly out.