This video shows where to place MiP's tray and explains the fun stacking game you can play with MiP. When in Stacking Mode, MiP's chest will light up pink.
In this game you will have only a few seconds to stack a heavy object or many light objects on top of the tray.
If it can balance correctly, MiP will add additional time for you to stack.
If you fail to reset the timer and the clock runs out, MiP spins around, tossing all the stacked items off the tray.
Video Tutorial Below.
Do not place open liquid containers on the tray. Do not place anything too heavy on the tray.