Meaning trap

act up

misbehave

to behave badly or not follow the rules, especially for children or pets.

phrasal verbB1common

Don't mix these meanings

Same phrase, different job.

misbehave

to behave badly or not follow the rules, especially for children or pets.

act up (not work properly)

Another meaning of 'act up' is 'not work properly'; compare the examples to keep the meanings separate.

Notice where the phrase sounds natural.

The kids started to act up as soon as the teacher left the room.family
When my little brother acts up in public, my parents usually try to calm him down quickly.IELTS speaking

Pattern: subject + act up

WrongThe kids are acting out again.

RightThe kids are acting up again.

For kids or pets misbehaving, use 'act up' (BrE 'play up'); 'act out' means expressing feelings through bad behaviour.

Patternsubject + act up

Choose the version that sounds natural in speech.

Stiff The children behaved badly and disobeyed all evening.

Natural The kids were acting up all evening.

'Act up' is the natural casual verb for kids misbehaving; the formal paraphrase sounds like a report.

Words that usually travel with it.

Don't confuse with 'act out', which means to express feelings through actions.

act up (not work properly) — Another meaning of 'act up' is 'not work properly'; compare the examples to keep the meanings separate.