Meaning: trifle with, toy with, confuse somebody.

Example:
1. It's just the newest trap in the everlasting cat and mouse game between the guards and thieves.
2. Don't play cat and mouse with that little child.

Meaning: to let a secret out in the open. To allow something that is hidden to be revealed.

Example:
1. The Mason's let the cat out of the bag at the end minute. Until then nobody at the party knew about the performance by such a famous film star in a regular party.
2. I have let the cat out of the bag, there is no point pretending that this job is working out for me.

Meaning: thinking of some impossible task. Make plans or hopes that have very little chance of happening.

Example:
1. She keeps talking about her big-time ambitions, but it's all castles in the air.
2. Don't build castles in the air, just and find some work to earn money.