Thought is the type of magic which allows the brain to think. In the Department of Mysteries of the Ministry of Magic, they study thought by keeping several human brains in a water tank in a room known as the Brain Room. Occlumency is a type of magic that involves clearing your thoughts of all emotion.
In the Battle of the Department of Mysteries, Dumbledore's Army and the Death Eaters fought in a room in the Department of Mysteries with a tank full of brains known as the Brain Room. A Confunded Ron Weasley thought the brains were funny and summoned them. After the thoughts soared at him, they started to wrap themselves around Ron's arms, nearly suffocating him. When the battle ended, Madam Pomfrey said that it would leave a scar since nothing leaves deeper scarring than thought.
- ""Hey, Harry, there are brains in here, ha, ha, ha, isn't that weird, Harry?...Honest, Harry, they're are brains — look — Accio Brains!" The scene seemed momentarily frozen. Harry, Ginny, and Neville and each of the Death Eaters turned in spite of themselves to watch the top of the tank as a brain burst from the green liquid like a leaping fish...The moment they came in contact with his skin, the tentacles began wrapping themselves around Ron's arms like ropes..."Harry, it'll suffocate him!" screamed Ginny."
- —Ron's attack by thought[src]