Two graduate students from the City University of New York's NOAA-CREST program showed me this simple experiment, above, for young kids. The three of us volunteered at an Earth Day fair at a New York City elementary school on Friday, and kids were mesmerized by it.

It illustrates the concepts of accretion -- when the tiny droplets of water that form clouds bump into each other and combine to form larger drops -- and cohesion, the attraction that water molecules have for each other. The "saturation point" that Isabel mentions in the video is the point at which a cloud can no longer absorb any more water and may release it as rain.

You'll need:

1. Wax paper

2. a spray bottle (optional)

3. toothpicks

Lay a piece of wax paper flat on a table or counter top. Spray it with the spray bottle, creating a field of water droplets of various sizes. Or just dip your finger in a cup of water and let several drops fall onto the paper. Let the child move water droplets around with the toothpick until they bump into and merge with other drops. At the Earth Day fair on Friday, kids had fun making the largest drop they could. You can explain that the larger the drop, the more likely it is to hit the ground before evaporating and that the largest raindrop ever recorded was 6.8 mm in diameter. Drops larger than that are believed to break apart.

For a full explanation, see JVC's Science Fair Projects.


For other science project ideas, see Scientific American's Bring Science Home.

Thank you to CUNY graduate students Isabel Perez and Roya Nazari for their help.