Please Explain

Matt insists on passing on his obsession with stickers to Jillian. He has succeeded, and she loves to play with them, pull them on and off her clothes, and even puts them on her baby dolls. If she sees a small piece of paper that she thinks is a sticker she gets very excited. For instance, she noticed a small Babies R Us diapers coupon that fell on the floor as I was cleaning out my purse this morning and went crazy until I gave it to her. Imagine her disappointment when she found it was only a lowly piece of useless paper. Anyhow, the point of this post is that I cannot for the life of me figure out how she came to give stickers the name she has pinned on them--"nee nee". It is clear, consistent, and the only object she labels as such. I'm stumped!