A shape in the dark, a face in a cloud and a creak in the night can all make us stop and wonder what made them. Let's look for what made each one. People see faces in clouds, rocks and tree bark, because our brains are very good at spotting faces. Seeing faces in things that have no faces has a name: pareidolia.

Shadows can make a face too. In one old photo, a hill on Mars looked like a face because of shadows. How do we know it is only a hill? Sharper photos showed it, and there are dozens of hills like it nearby.

Shadows can play tricks outdoors as well. On a mountain, your own shadow can fall on fog or cloud below you. The shadow of a mountain climber on cloud can look huge and have a colored ring around it. It is only the shadow of the climber.

Some tricks are made on purpose. Long ago, on a stage, people made a see-through ghost. They used a sheet of glass and a hidden actor in bright light. The ghost was a trick, and the glass and the light did the work.

Now for the sounds you may hear at night. Houses make creaks and pops, and wood floors and pipes make them. Wood, metal and glass get a tiny bit bigger when they warm up and a tiny bit smaller when they cool down. Metal air ducts can pop when they warm up. Each of these is a small, ordinary thing that a person can check.

Winter air dries wood out, so the boards shrink and pull apart a little. That is why some floors make more noise in cold weather. So a face in a cloud is a cloud, and a creak is a house.

Words to know

• pareidolia: seeing faces
• shadow: dark shape
• climber: mountain walker
• ducts: air tubes