When a guy or girl sees a "shiny penny" (a hot guy or girl) & all their attention is focused on them until they see a newer, shinier penny (a new hot guy or girl) & they completely forget about the first penny.
Girl 1: "Josh is so hot. I think I really like him."
*Hot stranger walks by*
Girl 1: "HOLY CRAP HE"S SO GORGEOUS!"
Girl 2: "What about Josh?"
Girl 1: "Huh? Who's Josh??"
Girl 2: "You totally have Shiny Penny Syndrome."