I went to buy the Christmas tree. The seller asked: "Do you want me to put some feet on it or you want to put it up yourself?"

OK, lets try and spell it out here then, as understand some do not get the wordplay here involved. (and the filthy thoughts going through the mind of the tree buyer). Also potentially because not so many are shopping xmas trees anymore from local forest traders etc, as they used to do in cities or from their farmland as used to be the case. Not to mention all the many relying now on xmas trees made of plastic... But here comes the guy up to one of these tree sellers. And finds the xmas tree he would like to buy. Such a tree seller would then typically offer also to wrap the branches a bit into some netting they have at their sales stand there. (makes the transportaion home much much easier) And also as courtesy to the buyer (to the city slicker with no tools at home for DIY work and that sorts), the seller would also offer to mount a cross set of wooden boards across the bottom of the xmas tree trunk end, as that would work splendid for the xmas tree to stand solidly on for the next 10 days or so, when the buyer got it back home. This cross is called the feet of the xmas tree. But as many started to have their own preferred xmas tree foot of some better and more solid materials/mechanics, that also were easier to adjust the leaning of the xmas tree, then these sellers now a days asks if you still want them to put on those feet they have for cheap, or if you prefer to put it on some feet yourself when back home. In casual language the seller therefor asks the buyer if he wants the seller to mount the feet onto the tree trunk bottom right then and there, or if the buyer wants to put up the tree himself when he gets home. Here the buyer then twists the reply, as it sounds instead as if the buyer had understood as if the seller had asked as if the buyer had considered sticking the tree trunk up his own butthole. (for sexual pleasure and that sorts) But to this the buyer just says 'no' to that specific question and says that instead he had actually considered that the tree should indeed be put up for decorative purposes in the living room. So the joke is of course that indeed the buyer apparently is such a sexual divergent, that it even came to his mind that this was what the tree seller referred to and had asked him about. To which he then just casually replied No, and that is was to be put up instead in his living room... ...Some folks have some very weird pleasures in life... And working in the ER department, it can at times be surprising to observe what pops up on the X-ray display when folks come in with some butt pain that needs taken care of...