It took my 2-year old daughter about 4 seconds to notice and develop a fascination with how Mommy feeds her new baby brother. My 4-year old son, however, did not notice until the baby was about 5 days old. One evening, while I was nursing our son, his older brother's eyes widened and:
"Mom! What are you doing?"
"I am feeding your brother. This is how he eats. His milk is inside Mommy."
"You need to be careful or he is going to eat parts of your body."