Beavers & Dams.

As a house cools down in the evening, its pipes, metal ducts and roof shrink a tiny bit, and its wooden boards shift as the air changes. When one piece slides a little against the next, it makes a small creak or tick. It’s the house settling in for the night, the same as you.

Most fridges have a pump called a compressor that carries warmth out from inside, which is what keeps the food cold. When the inside warms up a little, the compressor clicks on and hums, and when it’s cold enough again, it clicks off. It takes turns like that all night, so the milk is still cold in the morning.

With your ear pressed against the pillow, you might hear a soft, steady thump. That’s your own heartbeat, a little push traveling through the tiny tubes near your ear with every beat. It’s been going all day, and now the room is quiet enough to hear it.

When wind rushes past the edge of a window, a wire or a corner of the house, the air peels off in quick little swirls, one after another. Those swirls shake the air, and you hear them as a whistle or a low hum. When the wind changes speed, the note changes too.

When a car drives by outside, its headlights shine in through the window and make a bright patch on the ceiling or wall. As the car moves along the road, the patch slides across the room and slips away. Light travels in straight lines, so when the car moves, the patch has to move too.

In a dim room, a coat on a hook or a fold in the curtains can look like a face. That’s because the brain is built to spot faces fast, so it finds them even in things that aren’t faces at all. Everybody’s brain does it, and scientists have a name for it: pareidolia.