According to my physics book a long time ago, Einstein formulated the following paradox of simultaneity (thanks to ModernAngel, Stealth Munchkin and liveforever for quickly pointing out the right word). I think it was part of his relativity theory.

The story goes as follows. In the centre of a train carriage, a light bulb is switched on. The speed of light is constant, so the light reaches the front end of the carriage at the same time as it reaches the rear end.

```|                     |
|<-------- o -------->|
|                     |
|_____________________|
O                 O
```

So far, so good. Note that inside the carriage, this will happen even if the carriage is moving, because the speed of light is always constant (even if you are moving).

But what if you are watching the carriage from outside? (Say the side walls are open, so you can observe the inside.) Since the carriage is moving, the rear end catches up with the light before the light can reach the front end of the carriage.

``` -  |                     |
-  |<---- * ---->        | -->
-  |                     |
-  |_____________________|
O                 O
```

* is the bulb's original position, where it started sending out light. The carriage is moving very fast. :)

So whether the light reaches both ends of the carriage simultaneously, is dependent on where the observer is (or rather, how fast she is moving), because the light speed is always constant compared to anything else.

That's what you get for messing around with physical constants... ;)