Julia Robinson Mathematics Festival

2011: Diffy Boxes
What happens when you place four numbers at the corners of a square and keep taking differences?

2010: Fibonacci
The Fibonacci sequence, using dominoes!

2009: Tiling with trominoes
Where can the hole be when you cover an 8x8 board with 3x1 rectangles?

2007: Multiplication table and tetrahedra
Multiplication table / Tetrahedron/ Pascal's triangle proof without words