What you notice is the small hole at the bottom and things ratling inside the puzzle. You soon discover that these mysterious things are just ball bearings and from time to time when you depress the cap, these balls fall down. Do they block the cylinder and provide you from opening the puzzle? Well, this is something you will have to figure out or discover when the puzzle is opened!
Actually I opened the puzzle with luck by depressing and shaking the puzzle. You do not really have clues about how to open it, even with the sound of the balls.
When it opens the cap pops up with a satisfaying sound, nice opening!
OK I managed to open it, I looked everything and thought I could have opened it again, so I closed the puzzle.
Damn it! I could not. Same random process as the first time: a bit of shaking, a bit of "cap depressing" and it opens after some time...
Actually I know what to do, but doing it is not that obvious (well, if you have a technique that allows you to open the puzzles in 30 seconds, I would be happy to hear it!).
I like a lot the puzzle because the mechanism is nice and it's a nice challenge for the money spent on it. Its size has an advantage: you can carry the puzzle easily with you.
The thing that I do not like it that the mechanism damages a bit the inside bottom of the puzzle. I cannot say more without spoiling...I am sure nobody likes spoiling, but I am also sure that most of the readers own and/or have solved this puzzle :-)