The human brain is all about interpretation. It gathers information coming in from the senses and interprets it as something meaningful to the person owning the brain. This might be a pain response to a hot surface or a pleasure response to an appealing visual effect.
Because the brain is so heavily reliant on being fed information from the senses, when you deprive it of this flow, strange things can happen. Spend some time in a sensory deprivation tank and you will likely start to hallucinate at the very least.
But, you can achieve a similar result with some every day household items. Grab your portable radio and tune it off station until you get a steady flow of static. Now, take a ping pong ball, cut it in half and tape one half over each of your eyes. Lie down next to the radio and wait.
After a little while the brain will start to get bothered about not receiving any meaningful information from your two most powerful senses and it will start creating things on it's own.
The effect is known as the Ganzfeld effect and while results will vary per person, most folk report seeing vivid hallucinations within a few minutes.
