The boys have this Berenstain Bears board book, "Inside Outside Upside Down", and it's first words are 'Going in, Inside, Inside a box...' and it continues talking about prepositions and stuff. Mason loves this book and it's lyrics popped into my head as I caught him doing this with G-ma Gena's doggy door...

Climbing Up ................................. Going In

Inside ................................. Getting Up
And then at the end of the book, the little Berenstain Bear exclaims "Mama! Mama! I went to town. Inside, Outside, Upside down!" And when Mason stood up and looked at me, his face said about the same thing!