Why do 2 crates pulled together have the same acceleration?

[Josh114]

Hi. I was given the classic problem of a woman pulling crate 1 on a frictionless surface with a force F, which is attached to crate2 behind it with a rope. crate 1=6kg, crate 2= 4kg, and it says crate 1 has an acceleration of 2.5m/s^2...
One question asks for the acceleration of crate 2, and the answer is 2.5m/s^2, but I'm confused as to why they have the same acceleration. Could it be proved with force diagrams or with math?
Thanks

Attempt:
None. It's a conceptual problem, so there's no work to show

 

[Nidum]

If the rope is of fixed length and pulled tight what is velocity of crate 2 compared to crate 1 ? Less than , greater than or equal to ?

 

[CWatters]

What happens if the acceleration is not the same for both crates?