will
Explanation
The Joke
A lawyer is reading a will aloud at a formal table surrounded by family members of the deceased. The will states: "To my darling daughter, I wish to leave the British Library's entire ancient manuscript collection." The daughter immediately responds, "Goddammit, Dad." The caption at the bottom reads: "Funtime Activity: Willing stuff that isn't yours."
The joke is simple and clean: the deceased father has bequeathed something in his will that he never owned and had no right to give away -- the entire ancient manuscript collection of the British Library, one of the world's great research institutions. His daughter's exasperated reaction ("Goddammit, Dad") suggests this is exactly the kind of thing he would do, and that she now has to deal with the aftermath of his absurd posthumous prank.
The Humor
The humor works on two levels. First, there is the pure absurdity of someone casually willing away a national treasure they have no claim to, as if it were a family heirloom. Second, the daughter's weary, unsurprised reaction implies this is not the first time her father has pulled something like this -- even in death, he is being difficult. The "Funtime Activity" caption at the bottom, encouraging the reader to try willing away other people's property, adds a layer of mischievous encouragement, as if this is a hobby anyone can enjoy. It is a joke about the gap between the solemnity of a will reading and the pettiness (or absurdity) that can lurk within one.