Kids like the challenge of making a wall that will not fall down from "magnet sticks."

They are made with button magnets, which are 1/2 inch diameter disks, about 1/8 inch thick. The magnets are glued into holes in plastic or wooden sticks. Three sticks are about 6"x1/8"x3/4". One is long enough to form a diagonal, and two short stubs are glued into a piece of wood that forms the base of the wall. The base is attached to the shake table with Velcro. 

The task is to build a "wall with a flat top" that can withstand earthquakes. Typically the three 6" pieces are quickly used to from a wall, but it often takes a while for them to use the longer stick in as a diagonal brace.  This is the only orientation for which the wall is stable.

Here is a diagram of the four sticks and the base: