Watch this one together

~20 min · materials: build your own: a small plastic bottle, a clear straw, play-dough or sticky tack, and food-coloring water. fill the bottle to the brim with the coloured water, poke the straw halfway in (don't let it touch the bottom), and seal the neck tight around the straw with play-dough so the water rises a little way up the straw. hug the bottle with warm hands or stand it in warm water and watch your red line climb; stand it in ice water and watch it sink. mark your own degree lines with a pen.

How does a thermometer know how hot it is?

Run the fever thermometer under warm water (ask a grown-up first!) and watch the red line climb all by itself — nothing inside but coloured juice and physics. Then stand it in a glass of ice water and watch the line sink.