A nun is a woman who has taken vows to devote her life to a religious cause. Nuns usually live in a special place like a monastery, away from mainstream society. Nuns wear a special uniform, so you can tell who they are.
Onions are a vegetable. There are many different kinds of onions, ranging in size and colour from purple to red to white to green.
Onions are used in almost every kind of food, raw or cooked. They can be very sharp when they are raw and many children do not like raw onions, but when they are cooked they are quite sweet.