Papa and Grandpa?

I just love the toddler age as little ones begin to talk. They say the cutest things and some times they get things mixed up. My mother-in-law has been trying to get Mary to say Grandma for a long time. Mary is very happy to see her whenever she comes with lots of smiles and squeals but she has not called her anything for a long time. She said Mama at 7 months of age and said Daddy shortly after that. Then I think around 18 months she started calling Rich's dad, Papa. We would try and coach her different times to say Grandma or Nana but to no avail. Yesterday, his folks came by and picked Mary and I up and we went to a fairly new Walmart Super store that I had not been to. We had a lot of fun shopping, then we went to Carl's Jr for lunch. Mary and I had a great time. When we got home, it was Mary's nap time and she was very tired. His folks brought in all my stuff as I was getting Mary settled for her nap. When she realized they were going she got really upset. When they kissed her goodbye and closed the door, she went into wales of dismay. She sucked up her tears for a moment and looked at me and said, "Where Papa and Grandpa go? I felt bad for her because she was so sad but I couldn't help but chuckle at her. It doesn't matter how many times I correct her, she still calls Rich's mom, Grandpa! I know it is just a faze she's going through but it's just so funny, because his mom has tired so hard to get her to call her Grandma:)