You don't underestimate
"Head over heels in love" would be one idiom.
Head over heels is an idiom because the meaning does not match what the words are saying.
One idiom describes being in love as being "head over heels."
"Your head is going to explode" IS an idiom. It means you have too much to think about.
You could draw a picture of someone standing in a swimming pool, with the water level being above their head.
An idiom is something that does not mean what the phrase says literally, so yes. You can't actually laugh your head off.
Dozing off ; Being distracted ; Not paying attention
This is not an idiom. "It's over" means that it is over, or finished, or done. Whatever "it" refers to has concluded.
The original idiom was HEELS OVER HEAD -- which should explain things to you! It means you're so happy you're turning cartwheels.
The head person.
food
Over there.