Conan frowns in thought. He'd spent so much of his life within the world of detectives and detective novels that it was difficult to explain what seemed so obvious to him.

"Think of it like this," he says at last. "A crime is committed by someone. The culprit doesn't want to be caught, so they try and hide the evidence of the crime. Sometimes they create fake evidence so that the police will think that the crime was done by someone else. But no matter what they do, the truth is still hidden there. It is a detective's job to know what everything should look like, so they can figure out what is missing or shouldn't be there."

He frowns as he tries to think of an example that didn't involve murder.

"For example, lets say that some of the evidence of the crime was in a picture hanging on the wall. The culprit would remove the photo to keep the police or the detective from knowing it was there. But when a photo hangs on the wall for a long time it leaves a mark there from where it prevented sunlight from reaching the wall. If you pay close attention, you can see where something has been removed. So a detective would ask who would benefit from removing the photo and where it is now, and that becomes the key to solving the case."