Explaining breastfeeding and breast milk?

My 2 sons are 5 and 6 years old. This morning they accidentally stuck an elbow into my boob, something that normally doesn't bother me, but this time it hurt and I asked them to stop because it hurt. Naturally, they asked why. I told them because my boobs were starting to make milk for the baby. Then they asked how. I was unsure how to explain this to their age group. So any ideas or advice would be awesome. Thanks in advance.