It's like trying to get blood out of a stone.

He gets blood from a stone.

He tried to get blood from a stone.

Getting blood from a stone is nearly impossible.

Dripping water hollows a stone.

I always try to kill two birds with one stone by multitasking.

She kills two birds with one stone.

I always try to find ways to kill two birds with one stone.

I killed two birds with one stone.

He continues to beat his head against a stone wall, even though he knows it's futile.