Ted: MacLaren's Bar, four years ago...
[flashback to MacLaren's four years ago. Ted and Barney are both wearing tuxedos:]
Barney: How do you keep a girl from becoming your girlfriend? Simple, the rules for girls are the same as the rules for gremlins.
Ted: Gremlins?
Barney: Gremlins. Rule number 1, never get them wet. In other words, don't let her take a shower at your place. Number 2, keep them away from sunlight. I.e., don't ever see them during the day. And rule number 3, never feed them after midnight. Meaning she doesn't sleep over and you don't have breakfast with her ever.