The electromagnetic waves inside the microwave form standing waves, with nodes where no electric field would occur and antinodes where the electic field would be doubly strong. That's why, in the dark ages before microwaves had carousels to move the food around, you'd end up with one part of your pizza burning hot and other parts still frozen. These nodes form at half-wavelength intervals, so if you can find the nodes, you can find the wavelength of microwaves.
We decided to start with a plate of marshmallows. Hopefully, they'd puff up at the antinodes and stay the same size at the nodes.
The graham crackers help reduce reflections from the bottom of the microwave.