A floor creaks in an empty hall, and a shape in the dark seems to have a face. Stories about spooky places often begin with a moment like this. Someone sees, hears or feels something they cannot explain right away. A report tells what a person experienced. Evidence asks a second question: what could have caused it, and can anyone check? A society was founded in London in 1882 to study such reports with care, and houses, shadows, brains and sound all have something to say about the answer.

Houses make creaks and pops. Wood, metal and glass get a tiny bit bigger when they warm up and a tiny bit smaller when they cool down. 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. Cold pipes can make noise when warm or hot water runs through them. A draft is moving air that slips in through gaps around windows and doors. Wood floors can creak more in winter, because winter air dries wood out, so the boards shrink and pull apart a little. A noise that no one is making can still have a plain cause.

Seeing faces in things that have no faces has a name: pareidolia. People see faces in clouds, rocks and tree bark, because our brains are very good at spotting faces. Babies are attracted to faces from birth, even though their eyesight is poor. The brain area that notices faces gets busy when we only think we see a face, like one in a cloud. Pareidolia happens in the person who is looking, and it can happen to anyone who looks at a cloud or a piece of bark.

Shadows can make a face too. A hill on Mars looked like a face in one old photo because of shadows. The hill is 1.5 kilometers (one mile) across, and the Viking photo was taken with the sun about 20 degrees above the horizon. NASA says the hill looks like a face only in low-resolution pictures with very specific lighting. In other words, the face came from the light and the picture, and not from the hill. The hill has dozens of hills like it nearby, and sharper photos showed that it is just a hill.

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. The Brocken spectre is an observer's shadow cast on fog or cloud, often with a ring of colored light around it called a glory. A spectre is a figure that seems to appear from nowhere, and in this case the figure is the shadow of the person who is watching, with a ring of light around it.

Long ago, on a stage, people made a see-through ghost with a sheet of glass and a hidden actor in bright light. The trick is called Pepper's ghost: a bright light on a hidden actor, and a big angled pane of glass, so the audience sees the actor's reflection on the stage. John Henry Pepper ran the Royal Polytechnic in London and turned Henry Dircks's idea into a stage effect in 1862. The audience saw a reflection in glass, and the effect was built on purpose.

People generally hear sounds between 20 Hz and 20,000 Hz. Infrasound is below that range. It is sound with a pitch below 20 hertz, lower than the sounds people usually hear. Infrasound can be sensed if it is strong enough, mostly by the ear. A scientist named Vic Tandy traced eerie feelings in a lab to a faulty extractor fan that made very low sound. The fan was the cause he found.

The Society for Psychical Research was founded in London in 1882, to study such reports with care. Its first president was Henry Sidgwick. Scientists in England and Scotland have also asked visitors to walk around places with a haunted reputation and mark where they felt anything unusual. A mark made by a visitor is a record that someone else can look at later, and that is the difference between a report and a record that can be checked.

A 2001 Gallup poll found about 38 percent of Americans believed ghosts exist. In a June 2005 Gallup phone poll of 1,002 adults, 37 percent said they believe houses can be haunted. A poll shows what people believe. It does not show what caused a creak, a shadow or a feeling in a room. A person who reports a spooky feeling did feel something, and nobody has to be mocked for that. The evidence asks only where the feeling came from, and a creak, a face in the bark or a hum from a fan are all places to begin looking.

Words to know

• pareidolia: seeing faces
• infrasound: very low sound
• expand: get bigger
• contract: get smaller
• spectre: ghostly shadow