Spike realises there's a pretty noticeable gap in Twilight's otherwise encyclopaedic knowledge - and tries to fill her in. Confusion and embarrassment ensues (of course - this is Twilight we're talking about!).
(Teen rating and sex tag for discussion about sex and pony genitalia, no actual sex.)
EDIT: Featured on 19/3/19 to 23/3/19 - a huge thank you to you all!