Safety:
Be careful with the hot water!
Also, when displacing the tray of jello, it might be a good idea to put some paper towels down.

How to do the experiment:
Mix your jello solution (as per directions on the box).

Pour the jello into the pyrex tray. If you wish you can 'predraw' an XY coordinate system under the tray.

Align the pyrex tray with the X and Y axes.

Displace the tray in a diagonal motion - note the waves parallel to the X and Y coordinates created by the single diagonal motion.

Try deflections in different directions, with varying amounts of force (but don't spill it all over the place!). The height of the wave in the X and Y directions will be proportional to the component force in each direction.

When finished, or hungry, place in refrigerator to chill.

Explanation:
Since the tray is square or rectangular, when you displace the tray, the flat edges generate the waves against the inertia of the water. A single diagonal movement moves both the 'X' edge and the 'Y' edge to create the two waves within the fluid.

Useful References:
Your introductory physics or math text which describes vectors.

Further comments:
Serve with fruit or cool whip as needed.