Ever wonder if fanning yourself on a hot day does any good? In other words: Does it really cool you down, or, does the act of fanning yourself use up so much energy that it actually make you warmer? According to astrophysicist Dr. E. Phinney, the answer is “yes” – fanning works. And here’s why:

He says when you fan yourself, cooler, drier air molecules collide with your skin, and whisk away some of your body heat and sweat in the process. Dr. Phinney says the key is to keep the air moving, because the faster it moves, the more it conducts heat and moisture away from your body.

So, what about the energy you’re using to wave the fan? It turns out, Dr. Phinney says the act of waving does make you about 1% warmer than when you’re completely at rest, but that’s not enough to offset the fact that waving a fan doubles the air speed around you. In other words: In exchange for that 1% of extra energy, you’re cooling off twice as fast.