Cool puzzles. Mainly the last one. It gave me a while but I feel accomplished after solving it. Different possibilities came to my mind but after trying it execute it was clearly not it (like the fact that the magnet is not a solid object and can't hold the box over the button). Last puzzle is definitely my favourite.
Only problem I had was in the first level where I didn't know that magnets can go through the metal bar. I felt a little bit cheated. It would be maybe good idea to introduce them in the introduction level too.