1) let the cat out of the bag.
=> If you're holding a bag, and it's wiggling, people will be curious to know what's in the bag. Replace the cat with information, and you get the meaning in 1).
2) spill the beans.
=> same as 1)
3) not all there.
=> If the lights are on in your house, the assumption is that you are home, there in the house. If the lights are not on, the assumption is that you are not there, not at home. To be not there at all, or not at all there refers to a person's mind or cognitive skills. The person is standing in front of you, their eyes are open and looking at you (i.e., the lights are on) but you can't seem to communicate with them. They are there physically, but not all there cognitively.
4) to put on a brave front.
=> to be brave when you're really scared
5) water under the bridge
=> When water passes under the bridge, it's gone and no longer thought aboutr. 5) means, to allow yourself and others to forget about something negative that has been said or done.
6) to be a nit wit.
=> 'wit' refers to being smart. 6) is the opposite of that.
By the way, there are more than a dozen sites online dealing with Idioms. Check 'em out.
- For Teachers