I just watched a documentary about beavers.

This joke is a masterclass in wordplay, specifically a pun that hits you with a delightful linguistic surprise. The humor comes from the setup creating an expectation about a nature documentary, only for the punchline to twist a common exclamation into something hilariously literal.

The magic word here is "dam." While it sounds exactly like "damn," an expletive often used to express strong positive feelings (like "that was a damn good show!"), in this context, it cleverly refers to the actual structures beavers build. Beavers are famous for their incredible engineering prowess, constructing elaborate dams to create their homes and control water flow. So, a documentary about them would indeed feature a lot of "dam" material. The joke plays on this double meaning, taking an enthusiastic review and giving it an unexpected, beaver-centric twist that makes the whole thing a "dam" good laugh.