You need to ask him or her this question. As a parent, you need to be the authority figure. You need to let them know your love is unconditional, and they need to know they can tell you the truth no matter what. This is a difficult line to walk but a very important balance to establish. This depends upon your daughter's age and upon her need to know.
Children are ready to learn about sex at different ages.
The most important thing is to follow their cues, answer their questions and answer only their questions. Many times parents feel obligated to provide much more information than the child is actually requesting. Seek guidance from your pediatrician if you feel uncomfortable. Another great resource is the library. I suggest parents go alone and review books that teach about sex and physiology.