Waking up is hard, but for some of us, getting out of the shower in the morning is harder. Things are dandy at first: You step into the basin and happily roast under a steamy blanket of hot water. But then you shut off the shower faucet, open the curtain, and whoosh! You’re a human popsicle. Why does stepping out from a warm shower make your bathroom feel like Antarctica?

The answer is evaporation. When you step off the bathmat, the water clinging to your skin starts to evaporate. But to change into a gas, that water needs help—namely, it needs heat energy. It acquires that energy by sapping heat from your surroundings. In the case of your morning shower, the evaporating water sucks up heat energy from the droplets that stay clinging to your body. The result? The water on your body cools—and so do you. (The water glistening on your skin isn’t the only thing that gets icy. Evaporation also absorbs heat from your skin, making you shiver even more!)

Evaporative cooling may be annoying when you take a wintertime shower, but it’s handy during the stifling summer days. It’s what makes sweat—your body’s cooling mechanism—work. Of course, sweat doesn't always cool you down; it can be useless on a muggy afternoon. The air is so saturated with water that your sweat can’t evaporate, leaving you hot and soggy.

Which explains why stepping out of the shower can be such a jolt. The cocoon behind the curtain traps a lot of water vapor, keeping the air in the shower moist and warm. But the air outside the curtain isn’t as humid. So when you step out of the shower, you enter an environment primed to make the water on your skin evaporate quickly. Add in the fact that warm water evaporates faster, and your bathroom can feel like an icebox.