Thought Rattle is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target opponent reveals each nonland card in their hand. You choose one of those cards. Exile that card. Threshold — If seven or more cards are in your graveyard, seek a Rat card. It perpetually gains "This spell costs less to cast." You gain 1 life for each Rat card in your hand.