The last level in each chapter is a boss fight. You'll need to use all the skills you've learned in the previous 20 stages, but now while being chased by some big baddy.
Plan ahead
Remember that it's totally possible to mess up a puzzle completely if you don't plan ahead. If you do get stuck, hit the 'reset' button in the pause menu.
Toilet rolls
Toilet rolls are a bonus item for puzzle aficionados, but you'll need to find a certain number of them to unlock the next stage.