Talk until your child understands. Be clear about your own values. Children around 4 or 5 years old are becoming more curious about sex. Your pre-schooler may want to know where babies come from. He is asking lots of questions about everything as he is busily exploring the world around him.
Sexuality is just one facet.
Make your answers short, but concise. Your child doesn't need or want technical information. He will take one bit of information, think about it for a while, and then come back to you with another question. Tell your child that home is the proper place to discuss sex. If your child is five and still hasn't started asking any questions about sex, it's a good time to open the door to communication.