What is an idiom or expression for a hopeless task?

Herding cats
Nailing jelly to the wall
A Sisyphean labour
Rearranging deckchairs on The Titanic
Knitting fog

An exercise in futility.

"It's like trying to teach a pig to sing."
or
"It's like trying to teach Donald Drunk to tell the truth."

Trying to sweep out the incoming tide with a broom.
Finding a needle in a haystack.
If you are doing a creative writing assignment find a new phrase like:
"It was as hopeless as hiking to Mars in three days!"

To play the piano to a cow - Chinese idiom

When the hell freezes.
catching the wind with a net.
devil's dream in heaven