nice to meet you

Mabel wanted to sleep in Stella's bed tonight. Only once before have they been able to successfully fall asleep in the same bed together. Things generally start out nice and quiet with their heads at opposite ends of the bed. But after a few minutes that starts to deteriorate into giggling and poking and steamrolling each other until it becomes full blown laugher and there is no hope of sleep in the near future. Tonight we gave it another shot. Mabel was really insistent, so I gave in. They both like to have their backs rubbed in their own ways: Mabel likes to have her back actually rubbed and will say "rub my back" and then turn over on her tummy to make it happen. Stella likes to say "rub my back" but then when I start to rub her back it's too tickley for her so she just says no and pushes my hand away or turns over so her belly is up. After rubbing their backs for a bit I stopped and let them try to fall asleep. They laid quietly for a few seconds and then Mabel stuck her hand out across the bed and told Stella to hold her hand. Stella reached over, grabbed her hand and said "nice to meet you". This prompted Mabel to say "nice to meet you" back, which was followed by lots of giggling and more nice-to-meet-yous back and forth. And I couldn't stop laughing either. Where did they get that from???