Temperature is a measure of energy. Adding energy to a substance makes it hotter; removing energy makes it colder. Warm, energetic molecules move faster and farther, spreading out over a larger volume of space.
TeaRocket1
This balloon has been cooled to hundreds of degrees below zero (Fahrenheit), condensing the gas molecules inside. At room temperature, the condensed gas spreads out and expands, stretching the balloon back out to its original size!
We can make a gas less dense by heating it up. Less dense substances float in denser substances. This is how hot air balloons work! The warm gas inside is thinner and lighter than the air outside, so the balloon rises up through the thicker, heavier air around it. In this experiment, we’ll harness the temperature-dependence of density to turn ordinary tea bags into miniature rockets.
TeaRocket2
Step 1: Cut the staple, string, & folded paper away from the top of the tea bag. Step 2: Empty & unfold the bag to form a cylinder. Step 3: Ignite the rocket from the top.
Tea bags work well for this demonstration because they’re light, flammable, and conveniently shaped.  Emptying and unfolding the bag yields an open-ended cylinder. As the delicate paper burns, the air inside the cylinder heats up and becomes less dense. At the same time, some of the tea bag is converted to smoke, leaving a super-light skeleton of ash behind. Takeoff occurs when the structure becomes so light– and the air inside so thin– that the rocket is, overall, less dense than the air around it.
TeaRocket3
WARNING: flaming tea bags follow unpredictable flight patterns. If you try this experiment at home, be sure to choose a non-flammable setting, and keep a fire extinguisher handy.

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