N8N wrote:
Real cinder blocks do not make good makeshift jackstands, countless
redneck jokes notwithstanding. It's rare, but possible that they may
crumble under the concentrated weight of a car frame sitting on them,
which would be bad if you happened to be underneath the car at the
time and still inconvenient even if you weren't.

I suppose it could happen, but I've seen cars on cinder blocks, in someone's
front yard, for DECADES and the blocks haven't crumbled yet.