Everybody knows about the twin paradox. If a twin takes a trip in a spaceship around the galaxy while the other twin remains on earth, the twin who travelled will be younger than the twin who stayed home when he comes back.
The paradox is why the twin who travelled is the one to gets younger? Why not the one that stays at home? The answer is that the twin in the spaceship suffers the acceleration, so he is the one who gets younger.
Ok then. Now let's see another problem. Let's take two spaceships. Each twin is in one spaceship. They are approaching in a constant speed (close to the speed of light), with no acceleration. According to special relativity, twin A will se twin B getting youger, because in respect to twin A, twin B is moving. But, for twin B, twin A is the one getting younger, sice there can be no preffered referential. Hum, isn't that weird?
Now the problem itself: let's say that they are both smoking. They smoke at the same speed. Twin A smokes two cigarretes and sees twin B smoking one, due to time compression. Twin B smokes two cigarretes and sees twin A smoking only one, no preffered referential. Let's say now that when they cross each other in space, twin A throws his ash tray inside twin B spaceship. What will twin B see when he compares his ashtray with his brothers ash tray?