I was a breastfed baby and I can clearly remember visiting a neighbour's house and seeing my little friend being breastfed. My mom told be that she was drinking milk and I remember thinking that she must be shaping her mom's breast into a cup shape for it to hold milk!
I believe that one of the best ways to explain breastfeeding is to allow a child to see it. Breastfeeding has not been featured on many (or any) kids shows in recent years so I've posted two classic children's shows that explain breastfeeding at a child's level. Enjoy!