Random letter block will drop down to bottom step by step, just like the tetris game, use left and right arrow key to move the letter block, use space or down arrow key to accelerate block dropping. When the block drop to the bottom, the correct word(3-8 letters) will be moved, and the above blocks will drop to the bottom. The correct words will be listed in the box on the right. If the new letter block can't drop, game over.