This was probably asked some time ago. It involves a paradox of relativity and I hope someone can answer it for me:
Imagine a solid cylinder (call this the plunger) with a bar welded to one end making a T. The plunger fits just perfectly into a hollowed cylinder of the same length, and the end of bar welded on the end prevents the plunger from going in any further. There is a button at the end of the hollow cylinder that, when pressed, triggers a bomb. It is just far enough from the fully engaged plunger so that it does not get pressed.
Now if we send the plunger forth at the speed of light into the hollowed cylinder, someone standing next to the hollowed object will see the length of the plunger contract. When it enters the cylinder it will be too short to press the button. However, someone riding on the plunger will see the hollow cylinder contract. Its length is contracted enough that the plunger can reach the button, and detonate the bomb.
So what happens? Both can't happen.