Sick jokes form a continuum, from those which are actually funny if a bit twisted, to those which are sick for the sake of being sick.
Mommy, Mommy, why am I running around in circles?
Shut up or I’ll nail your other foot to the floor.
has a genuinely clever idea behind it, even though someone who can’t help but think about the ugliness of the implied situation is going to be repulsed rather than amused. Not that there’s anything wrong with that. (If you think, “If she tied his feet to a loop of rope wrapped around two nails, he’d run around in an ellipse”, you should seek professional help.)
This one, on the other hand, rather than being clever, exists only for its shock value:
What’s the difference between a truckload of rocks and a truckload full of dead puppies?
You can’t unload the rocks with a pitchfork.
And is generally told by teenagers impressed by their own daring in doing so.
Make up your own minds about this last one, ROT13’d for your protection:
Jung qvq Wrssrel Qnuzre fnl gb Yberan Oboovg?
“Qba’g guebj gung njnl, V’yy rng vg.”