Let's look for what could make a ghost seem to appear. A ghost walks across a stage, and you can see right through it. Long ago, people made a see-through ghost on a stage, using a sheet of glass and a hidden actor in bright light.

The trick is called Pepper's ghost, and here is how it works. A bright light shines on a hidden actor, and a big pane of glass stands at an angle. The audience sees the actor's reflection, so the ghost is only a reflection. John Henry Pepper ran the Royal Polytechnic in London, and in 1862 he turned Henry Dircks's idea into a stage effect.

Some tricks happen in the brain. People see faces in clouds, rocks and tree bark. Our brains are very good at spotting faces. Seeing faces in things that have no faces has a name: pareidolia. It is a long word for a face that turns up where no face is.

The brain area that notices faces even gets busy when we only think we see a face, like one in a cloud. Shadows can bring that on, too. In one old photo, a hill on Mars looked like a face because of shadows. Later, sharper photos showed it is just a hill, and there are dozens of hills like it nearby.

Now think about the sounds a house makes 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. Cold pipes can make noise when warm or hot water runs through them. Metal air ducts can pop when they warm up.

The time of day and the seasons matter, too. Building materials in a house, such as wood, steel, masonry and glass, expand in the heat of the day and contract again at night when it is cooler. Winter air dries wood out, so the boards shrink. That is why some floors make more noise in cold weather, because dry air makes the boards pull apart a little. A draft is moving air that slips in through gaps around windows and doors.

Light can fool the eye outdoors, too. On a mountain, your own shadow can fall on fog or cloud below you. A mountain climber's shadow on cloud can look huge and have a colored ring around it. It is only the climber's own shadow. It is just a shadow, even when it looks huge.

Words to know

• pareidolia: seeing faces
• reflection: mirrored picture
• shadow: dark shape

The short version

Let's look for what could make a ghost seem to appear. Long ago, a see-through stage ghost was only a reflection in glass of a hidden actor in bright light. Brains are very good at spotting faces, and shadows can make one. Houses creak and pop, and wood floors and pipes make them.

Quick facts

• 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.
• The brain area that notices faces gets busy when we only think we see a face, like one in a cloud.
• Pepper's ghost used a bright light on a hidden actor and a big angled pane of glass, so the audience saw a reflection.
• A hill on Mars looked like a face in one old photo because of shadows; sharper photos showed it is just a hill.