A bouncing ball on a train is quite easy to understand.

Why the very same ball following the very same motion over the very same time period in front of 2 people travels a different distance is much harder to intuit, even for a 5 year old.

People never exist in 2 places at once, so relativity is unintuitive.  But 2 things always exist in 2 plaves at once, so relativity is helpful in navigating our realm.