• An empty, clean 2 liter bottle with the label removed
  • Hot to warm water
  • Matches (have an adult present)


Fill the bottle 1/3 to 1/4 way full with warm to hot water. Put the cap on the bottle and give the bottle a squeeze. Observe the conditions under pressure then release. You’ll notice there is no change! You might notice a little condensation on the sides of the bottle but the air remains clear inside. Remove the cap and light a match. Hold the lit match over the opening and drop it into the bottle. Replace the cap. Repeat the squeezing process. You’ll notice when you squeeze the air clears and when you release a cloud forms inside!

Why it works:

Clouds need 3 basic ingredients. Water vapor, low pressure for best growth and cloud condensation nuclei. The hot water supplies the water vapor as it evaporates into the air in the bottle. The squeeze provides a pressure change from high (when squeezed) to low (when released) pressure. There reason why nothing happened when we first tried it is because we were missing our third ingredient, cloud condensation nuclei. This can be dust, ice crystals, pollen… any small particle really! In our experiment it is smoke from the match. This gives the water vapor something to cling to. Under high pressure we see clearing in the bottle. This is what we see in nature too. High pressure usually means a sunny day. When there is low pressure the atmosphere is less stable, we have more rising air, more clouds and stormier weather. When we lower the pressure in the bottle by releasing the squeeze we see a cloud form!