They shake your hand while smiling and says hello, then walks away.
Most Helpful Girl
Omg, this (not exactly this but you know), happened to me!
I just stopped walking, had a confused look and a small smile on my face. I was kind of surprised and didn't know how to react.
Once he walked away, I said in my head, "Gah, why do you do this? "
So my reaction would be, stop and stare and just freeze.1
Most Helpful Guy
Me: "Oh.. uh. uhhhhh..."
*stranger walks away*