I can't belive I didn't find this one here:
Achilles and the Tortoise is one of Zeno’s paradoxes. It appears to demonstrate that even the fastest of runners, Achilles, could not catch the slowest of creatures, the tortoise, if the tortoise were given a head start.
To get from one place to another takes time. If the distance between the two is very small, or the speed of travel is very fast, then it may take a very short amount of time, but it will take some time nevertheless. It is impossible to move from one point to another instantaneously.
To catch someone, you need to cross the dist