Open a can of worms.


 Here we see a pair of fish having a discussion at the bottom of the sea.  Fish love to eat worms, and one of them, named Frank, is tempted to open the worms with a can opener. He appears to have missed the fishing line attached to the can.
The other, smarter fish, is warning Frank that opening the can of worms could mean a lot of trouble. Frank should listen to his friend's warning, unless he wants to be somebody’s dinner! The idiomatic meaning of 'open a can of worms' is to create a situation which leads to trouble. Let's take a look at this idiom in a conversation:

Hey, Simon. Is Janine invited to your sister's wedding?
Sure. Why not?
Well ... Rhys is going to be there too.
So?
Isn't that going to open a can of worms?
What do you mean?
They haven't spoken to each other since they broke up!
Oh, you're right! And they often fight.

Hold your horses.



This is a tag-team wrestling match between a pair of elephants and a pair of horses. 
The elephants have the obvious advantage in size and strength, but the horses are too excited to realize this and they want to get in there and fight. Their manager sees the danger and wants to change his mind, so he’s holding his horses back from fighting the elephants. 
You usually say 'hold your horses' to tell someone to stop doing something because they are going too fast and could face a problem. Here's an example: 


I'm so excited about this concert!
Me too. I'm really looking forward to it. 
I hope it isn't sold out.
Two tickets for the front section, please.
Hold your horses, Mike. Have you seen the prices?
Oh, they're pretty expensive. 
Right. We won't have enough left for a taxi home.
Okay. Let's get cheaper seats.