I'm collecting advanced exercises in geometry. Ideally, each exercise should be solved by one trick and this trick should be useful elsewhere.

If you have a problem like this please post it here.

**Remark:**

 - I'm collecting such problems for many years. The current collection is [here][1], it is about 70 problems. At the moment, I have just few problems in topology and in geometric group theory and I do not have any problem in algebraic geometry.

 - Thank you all for nice problems --- I gave bounty to one, but would give it to 4 problems if I could :). Some of them are in the collection by now --- **please post more**...


  [1]: http://dl.dropbox.com/u/1577084/problems.pdf