act up (no object): misbehave (for people); not work properly (for machines).

"The babysitter had a difficult time. The children acted up all evening."

"I guess I'd better take my car to the garage. It's been acting up lately."

act like (inseparable): behave in a way that's like _____ .

"What's wrong with Bob? He's acting like an idiot."

Note: This phrasal verb is very informal.

add up (1. no object): logically fit together.

"His theory is hard to believe, but his research adds up."

Note: This phrasal verb is often negative.

"His theory seems, at first, to be plausible, but the facts in his research don't add up."

add up (2. separable): find the total.

"What's the total of those bills? Could you add them up and see?"

add up to (inseparable): to total.

"The bills add up to $734.96. That's more than I expected!"

ask out (separable): ask for a date.

"Nancy has a new boy friend. Joe asked her out last night."