Teapot Swirls

Here's something to notice when you're making tea on a cold day. If you pour cold and hot water together, you might notice swirls of light in the water. Not because the water is colored (it's not), but because the hot and cold water have different densities, and thus bend light in slightly different ways. Mix them, and you can see the fluid swirling as light bends at the boundaries between the hot and cold.