Even if you live alone, it’s easy to believe that a tiny man with a hammer takes up residence in your radiator all winter long, happily banging away. What else could explain those sudden, low-pitched clangs, which are as unpredictable as they are annoying? But as SciShow’s latest video explains, tiny drops of water are to blame for the phenomenon.

“You have a central boiler that boils water, and the steam that the boiler makes takes up way more space than it did as a liquid,” host Stefan Chin explains. “As it expands, it pushes itself through a series of pipes. Now, assuming you've opened the intake valve to let the steam into the radiator, it will rush in and push the air out through a small air vent on the opposite end. And when there's enough steam inside the radiator, the pressure causes the vent to close, trapping the steam inside.”

As this steam cools, the radiator’s metal grows warmer. Some of the vapor converts back into water and drips down the pipes, but the steam can also fling droplets of condensed water against the radiator’s inner walls—and that's what causes those startling sounds.

Watch SciShow’s full video below to learn more about why your radiator becomes louder than a mechanic’s garage during the winter.