What are twins?

Today, little Elijah (4yrs) heard the word "twins". I cringed when he asked, "Momma?  What's twins mean?" I reluctantly told him that it's when a Mommy has two babies in her tummy instead of one. "TWO BABIES!!!  A mommy can have TWO BABIES!?!??!  That's what I want! Come here right now...let's pray!"  HAHA!  NOW I've done it! hahahhaha!