手の内を明かしなさい。

palm (of one's hand); skill; scope of one's power; one's intentions; one's plan; one's hand (mahjong, card games, etc.)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to pass (e.g. the night); to spend; to reveal; to divulge

to do