  We consider a few thought experiments of radial motion of massive particles
in the gravitational fields outside and inside various celestial bodies: Earth,
Sun, black hole. All other interactions except gravity are disregarded. For the
outside motion there exists a critical value of coordinate velocity ${\rm v}_c
= c/\sqrt 3$: particles with ${\rm v} < {\rm v}_c$ are accelerated by the
field, like Newtonian apples, particles with ${\rm v} > {\rm v}_c$ are
decelerated like photons. Particles moving inside a body with constant density
have no critical velocity; they are always accelerated. We consider also the
motion of a ball inside a tower, when it is thrown from the top (bottom) of the
tower and after classically bouncing at the bottom (top) comes back to the
original point. The total time of flight is the same in these two cases if the
initial proper velocity $v_0$ is equal to $c/\sqrt 2$.
