Have you ever seen a snake in a jar? Open the top and it pops out. Well, take it out of your pocket and the thing pops up! The spring is insanely tight and this pup turns in a split second. A strong spring stretches the material well. It's a little tricky to close, but it gets easier with practice. Eat your spinach and get ready for a workout by putting it back in your bag. Don't get me wrong, this is a very positive review, I will buy again. Just be aware that it will burst the first time you open it!