Children aged under 14 in Jersey now have to wear a cycle helmet when out riding their bike.
It's after the introduction of a new law - the first time there's been such a law in the British Isles.
The law applies to riding a bicycle on a road or cycle track, and to children in a trailer that's being drawn by a bicycle.
Anyone caught without a helmet could be fined £50, but a warning is more likely.
Jersey is in the Channel Islands and has its own government separate from the UK's.