A fool's errand

To draw a bead upon

I <u><em>raked my brains</em></u> to solve this difficult problem.

At one's beck and call

To wrangle over an ass's shadow

You need rest; you look a bit <u><em>run down</em></u>.

To play the gallery

Why must you look a <u><em>gift horse in the mouth</em></u> ?

He can get the job if he <u><em>has the mind</em></u>.