How should you explain masturbation to a child when he or she asks?

Good idea to wait. Until and if child actually asks. You can then tell child that some people enjoy playing with their genitalia because it feels nice. Girls should be advised only to rub or stroke the clitoris and avoid inserting finger or objects into vagina. Older, postpubertal girls can be told about dildos, again only if asked. Boys can be told that masturbation ejaculation occuirs only after puberty.
Normal. Explain it as you would explain anything. Tell them the truth but use words that they would understand. Better to hear and discuss with the parent they find out from their friends.You don't need to go into depth just answer the questions as simply as you can.