Take your youngsters talks and concerns about sex very seriously and above all never misinform them. If their concerns are met with laughter they'll find it very hard to share anything sexual with you. Children want your wisdom and authority. Above all, your serious advice. A long speech seems like you're speaking at and not to your child.
Children tune off when they receive speeches.
So be informal, spontaneous and leave the door open for further discussion. It is important to remember that you must provide the opportunity to talk alone with each child. Take every question asked at face value without jumping to conclusions. If you're sympathetic and the child trusts you, the chances are that she'll tell you. Let's talk it over another time.