If you want to know just how sizzling it is outdoors on any summer day, all you have to do is look at a thermometer, right? While that's technically true, oftentimes there's a significant difference between how hot it actually is and how hot it feels. A 92˚F day, for example, can seem cooler than a day when the temperature tops off at 84˚F. It all depends on the relative humidity -- how much moisture is in the air compared to the total amount the air can hold. The hotter it gets, the more water the air can contain, and the more it contains, the hotter we feel.

To gauge relative humidity, meteorologists use an instrument called a psychrometer. Here you'll find a homemade version that's a snap to assemble and fun for your child to use. She just has to swing it through the air for a few minutes and then look up the readings she gets on the accompanying chart. For fun, she can keep track of how often her results match those of your local forecaster.