Lily's bisexuality is a recurring element in most seasons of the show.
Lily reveals that one of her unfulfilled dreams is having a lesbian experience, so Robin kisses her so that she can check it off her list. This likely begins Lily's attraction to Robin.
Lily admits to having "noticed Robin's body".
Lily mentions confusing dreams about Robin that remind her "a woman's sexuality is a moving target."
Lily mentions that she is reading Robin's "two-volume playbook", referring to her breasts. Also on that episode she refers to Robin as ''this hottie'' while talking about her recent relationship with Barney to Claire.
Lily says that Robin has '"crazy long legs that look great in shorts."
When Marshall says he can't stop thinking about Lily and Robin while in bed Lily responds by saying she's had that fantasy as well and tells him to "lean into it and let it fuel things."
Lily mentions having a secret crush on Mila Kunis.
Robin reveals that every time Lily drinks martinis she wants to make out with her.
Lily mentions she has dreams about Robin.
When Ted breaks a "Broath", Barney punishes him and the gang by making the guys and the girls kiss. They kiss and when Robin pulls away, Lily keeps on kissing her. Earlier in this episode when the gang is all at Quinn's apartment, Lily mentions that she was once with a dude who couldn't even fantasize about having a three way. "He was all, 'ooh she's your best friend.'" Then Robin asks, "what now?"
Lily fantasizes about a "slutty and bi-curious" Danish exchange student named Nadia.
When Robin attempts to hook up with Barney and fails several times, Lily suggests getting freaky with a girl to entice him. Lily is excited to get to grind with Robin but she brings Brandi (her fun coworker who doesn't even care) to the bar for that purpose, so Lily is disappointed.
When Lily knocks on the door of the girl wearing the t-shirt that says "Vagitarian" at Wesleyan, she smiles shyly and seems tempted to talk to her before she forces herself to continue looking for Marshall.
When Lily realizes Robin does not have many female friends, she attempts to help her by making her go up and comfort a random girl who is crying at the bar. After initially struggling, Robin manages to hit it off with the girl, who is called Amanda, over their shared love of ice hockey. Lily begins to feel jealous and has a vision of Robin and Amanda having an affair so she scares off Amanda when Robin goes to the bathroom.
In a hotel Lily has to pay for things that she didn't broke or asked for, when the receptionist asks "So you are saying that you didn't order room service and you didn't order Prison Sluts 9?" and she answers "I'm saying I did not order room service..."
Lily and Robin share a Telepathic Conversation about Darren probably wanting to hook up with them, during which Lily suggests that the two of them can continue after Darren is finished and Robin gets weirded out.
Lily and Robin end up kissing to get Barney to wake up after he drinks too much the night before his wedding. Later when they are at the bar, in a sudden role reversal, Robin is the one who wants to kiss Lily again but Lily says it was weird and doesn't want to do it again..