scratch one's head

/S K R AE1 CH W AH0 N EH0 S EH0 D/
verb
  1. 1

    To puzzle, ponder, or wonder about something.

    โ€œFaced with a box full of hundreds of small parts and forty pages worth of assembly instructions, he could do little more than stand there and scratch his head.โ€

Source: https://en.wiktionary.org/wiki/scratch%20one's%20head