Hanuman is shown with a bell on his tail.
When Rama assembled the monkey army he promised that they would all return to their families. Nobody would fall in battle.
Little monkeys also belonged to the army; they did not carry weapons, but fought with teeth and claws. Their strength was their appearance en masse. Some chariots were equipped with bells, one came loose in battle it fell on exactly a thousand of these little monkeys.
When the battle was over, Rama counted the soldiers. A thousand were missing. It was recounted. No change in the result.
Rama and Hanuman went to search. Rama saw a bell on the floor. Hanuman stuck his tail into the ring and carefully picked up the bell. The little monkeys were saved. Rama asked Hanuman to transfer all his power to this bell and leave it on his tail.
So today Hanuman’s tail is an object of worship, the devotee strikes the bell, expresses his request and it is fulfilled by Rama.