Tonight's zany Xander moment is brought to you by: There's really just one explanation that makes sense...

Daddy made a pineapple cake for dessert tonight.
Penny asked: Is it pineapple upside-down cake?
Xander laughs and mumbles in his just-turned-three gibberish: Daddy spanked the cake's bottom!

I thought about it for a second and realized that's a reasonable explanation of why a pineapple cake would be upside-down.