In the visible light spectrum, standard glass is almost completely transparent, and the metalloid germanium is almost completely opaque. But in infrared light, the opposite is true.
Here's another demo:
ProTip: if you find yourself hiding from authorities looking for you with an IR camera, your best bet is to find a large cardboard box and make sure you're not too close to the surface. Make sure it's well-vented so the heat doesn't build up inside!
Image: YouTube / Cody'sBLab