To get on the subway (or bus) and not see any attractive women. But within 15 minutes, you have lowered your requirements so much that the average-looking woman, when compared to the other women on the subway, is now very attractive. And you have decided that she is so wonderful that you can't stop staring at her, and even get a little aroused and lose focus.
I got on the F train on 1st and 1st. By the time I hit 53 and Lex, the chick with the mole between her eyes turned me on. I was in "Subway Love".