Backpack in Adventure Time

[MunchkinMan]

To be very specific about why: Nothing in your hand can ever do anything for you. The rules say, "Cards in your hand are not in play. They don't help you...." You must play, or put into play, any card for it to have any effect on the game. Otherwise, how are people supposed to know of a card's effects if you don't have the card in play for them to see its text? Since Backpack doesn't say it's usable once only, it must be in play before it can have any effect, it can't be played as a reaction to something, and if you were allowed to keep it in your hand to use its effects, you'd appear to be breaking the rules without people knowing how or why:

"I see that you have 10 cards in your hand."

"Oh, it's okay. You'll just have to trust me that I'm allowed to have as many cards as I want."