The picture above shows two arrangements of four rectangular blocks, each labelled A to D. The arrangement on the left is the starting arrangement, the one on the right is the goal position. To solve this puzzle you need to slide the blocks around so that the starting arrangement is transformed into the goal arrangement.
Oh, but there's a catch...
You need to solve the puzzle in 16 moves or under.
A move consists of moving a block to a new position so long as you don't hop a block over another block, twist a block around on the spot, move a block on top of another block, or move a block out of the frame.