It isn't polite to click at humans..

I very nearly missed my flight because the airport parking lot is full and I couldn't find the overflow parking. That problem was solved. Then some lady in the TSA line was walking really slowly and I was stressed out, so I clicked at her.

She turned around and glared at me and then said "just go ahead of me."

I just said, "Sorry. Thanks."

I didn't mean to click at her like a horse. I was just half panicked about missing my flight and she was walking so damn slowly.

It isn't polite to click at humans when you want them to move. But apparently it works.