Why does your mind play tricks?

Your mind doesn't "play tricks" like a person would do to their friends on April fools day. What your mind does do is try to make sense of the world around you and make sense of things out of what information it is given.

For example people see faces in all kinds of blobs and photos. This is known as Pareidolia.

When our brains see or hear things that make no sense, the brain tries very hard to turn it into something we do recognize. That is why we make out patterns in clouds, or certain words when a record is played backwards.

The mind is not a trickster out play jokes on you, but sometimes it does its job a little haphazardly, and we end up thinking we saw something we didn't.