When puberty kicks in = when puberty starts to change a teenager's body or behaviour. Example: Now puberty has kicked in, I can never get into the bathroom because my son's washing his hair.again. Example: Once puberty kicks in, you'll have to wear a bra. Fixed idea = an opinion which will be hard to change, even if it is wrong. Example: He's a man of fixed ideas; I've tried telling him that cheese is fattening, but he still thinks it's a healthy choice. Example: She had the fixed idea that her husband was gorgeous and every woman was after him, even though he was 5'4", ugly and stupid.

1. 'Kick in' is an intransitive phrasal verb meaning to show an effect. It is often used in the context of drugs. For example, when a painkiller kicks in, you feel its effects and your pain lessens. In this context, it means that the adolescent hormones are taking effect, and a young person begins to experience the physical, psychological and behavioural changes associated with puberty. 2. If a person has fixed ideas and prejudices they are incapable of thinking or using their imagination.