The birds and the bees

How do u explains the birds and the bees? So I had my son at 18 and I'm raising him as a single mom. Now he's 8 and asking if he was adopted because he thinks that all children are adopted so I decided to explain. I started with "if a woman and a man love each other," I looked into his eyes and chickened out and finally said, "it's a miracle" lol not my finest moment but any ideas how to approach the talk or should I wait till he's a little older?