How do you explain to a child where a baby comes from?
Age appropriately, you do not delve into the sexual act itself. Use something that on their level they can relate with. If they are able to comprehend this then for 3-5 try this. Get some seeds and a small pot of soil explain that a baby grows in mommys belly like a seed grows in the pot. Have them put the seed in the soil, water it and watch it grow. Explain how the seed needs water and sun as food to be healthy. If they are over 6-8, which unfortunately these days, they already have, even if misinformed, some idea from other children that get told/shown too much, too early, an idea but better to tell them your own way. Explain that a man and a woman have very special parts on the inside of their body that when get together, make a baby. If further inquiry, be honest and tell them mommy has an egg and only mommys have this and daddys have a fertilizer, and only daddy's have this. Of course that will lead to, how to they meet and the best way of explaining that is, "well, when mommy and daddy have private time, there is a special way that we show we love each other and when your older I'll explain that to you. If they ask if they can get pregnant,etc tell them that it doesnt happen until they are much older.
When answering a kids question, on where do baby come from, you have to know the level of understanding of the child. If the child is in grade school, in a way you can explain briefly and simply the process of conception, pregnancy and birth. If the child is below 5, they are more into imagination, so you can tell in simple yet easy to understand terms.