If you want to get a visual on what’s in your water, just pour it into an ice cube container and freeze it. Here’s why all the stuff that’s in your water aggregates in the middle, and how you can make it not do that.

Your water is filled with all kinds of things. Not all of them are bad. (And most of the bad things are probably more concentrated in your mouth, your sweat, and any droplets of water on your kitchen counter, anyway.) Mostly they’re things like dissolved air, dissolved carbon dioxide, magnesium, fluoride, and calcium. Whatever they are, they always seem to get stuck right in the center of any block of ice.

This is especially visible in the ice cubes you slide out of the freezer. The sides are crystal clear, but the centers are opaque. This is the result of the freezing process. City sidewalks get salted every winter for a reason. Salt lowers the freezing point of water. Almost any dissolved solid will do the same. But as water freezes, it rejects impurities. This is why, after very cold nights, people will wake up to partially frozen streets made extra slick by a sludge of salt crystals. In your freezer, your ice cube freezes from the outside in. As the sides and top freeze, impurities get pushed to the center, where they are trapped.

Fancy establishments, whose patrons are for some reason offended by the sight of cloudy ice cubes sometimes pre-boil water to get rid of the gases in it, but they also have special freezers. The freezers are kept at a much higher temperature than a kitchen freezer, and allow the water to freeze slowly. This gives any leftover oxygen a chance to escape. It also tends to freeze the ice not from the outside in, but from the top down. Then it’s just a matter of cutting off the impure end of the ice cube. If you’re feeling fancy, you can recreate the effect by putting your water in larger containers, putting those containers inside an igloo cooler that is also filled with water, and then freezing that.

Image: Kyle Flood