What's the difference between "Classical" and "Speed" mode?

You can choose between classical and speed mode. 

In classical mode you can solve as many puzzles as you want. You can decide to get feedback after each puzzle, after a wrong answer or after you finish the session. 

In speed mode you can select the duration of the session: one, three or five minutes. That means more time pressure! You have to solve as many puzzles as you can and get feedback at the end of the session.