Hardest modern concept to explain?

[BAndrew]

How gravity effects time. Literally slows down time the more gravity is placed on it.

clocks in space run just oh so very little bit faster then te ones on earth. They have to account for this in order to keep the times synced.

In other worlds general theory of relativity I don't think this will be THAT hard to explain. The whole idea is based on a different geometry and that spacetime is flexible. Of course it's mind blowing, but not something difficult to explain (well kind of).