Intersect two volumes in space and sculpt the shape that results from the collision. Each volume is made of a different material of your choice and the speed, angle and moment of collision are also determined by you. 

This is the assignment that I give all my second year students in our Form class. It's an introduction to thinking about 3-D form in a dynamic and tactile way. We use blue foam as the medium to sculpt the collision because it's inexpensive and easy to work with. Once they've done 5-10 sketch models, they begin to see form as an exercise in defining relationships between things, not just designing a single thing.

The crucial part of the lesson revolves around the dimensions of the two volumes. They have to start with one being 4" X 4" X 2" and the other being 2.5" X 3.5" X .125". Some of the students realize that the second one is approximately the dimension of a credit card yet even fewer make the connection that this assignment ultimately leads to a discussion about designing an interaction between a credit card and a machine (ATM, Gas Pump, Kiosk) where one would dip, swipe or insert their card.