In Box Challenge, you have to drop the block into the slot at the bottom of the screen. You have to prevent the box from tumbling through the tunnel at the bottom of the slot and dropping out of the level, so make sure to grow the box to the right size first. Watch carefully as you hold your mouse button or finger pressed down. The moment you think the box has reached the right size, you can release to drop it. Was your estimate spot on? Does the box fit into the slot, or did you make it too big or leave it too small?