Watch 'HIMYM' Welcome Another 'Liar'

In 2007, Lucy Hale played Robin's little sister, Katie Scherbatsky, in an episode of How I Met Your Mother, titled First Time in New York and on Monday, another one of ABC Family's Pretty Little Liars heads on over to CBS' hit comedy.

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In Ring Up, Ashley Benson plays a much (much!) younger girl Ted has started dating ... one whose date and word choices constantly remind him just how old he actually is.

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Watch ETonline's exclusive sneak peek of Benson on HIMYM and use the comments to guess which girl (Troian Bellisario, Shay Mitchell) will stop by MacLaren's next, making it a PLL hat trick!

How I Met Your Mother airs Mondays at 8 p.m. on CBS.