S01E03 The Sweet Taste of Liberty
Rather than go to McLaren's, their usual hangout, Barney convinces Ted to go to the airport with him to "pick someone up". It turns out that they're there to pick up some girls. While doing this, they fly to Philadelphia, get arrested by airport security and that's when Ted starts to wonder why he ends up in these adventures with Barney. Meanwhile Marshall has studying to do, so Lily and Robin do a girls night out and Lily notices guys are buying drinks and try talking to Robin, so Lily starts to wonder what it would be like to have her engagement ring off, and with Marshall's "blessing", she takes it off. In the end, Lily realizes that things aren't all that great with the ring off and Ted understands why he goes on these adventures/misadventures with Barney.