Ian Ayres of Yale Law School explains the concept of burden of proof, one lesson in his course A Law Student's Toolkit, a series aimed at giving incoming and aspiring law students the vocabulary and reasoning tools used in legal argument. This installment, the twenty-third in the series, covers who must prove what in a legal dispute and how different standards, such as preponderance of the evidence versus proof beyond a reasonable doubt, shift the weight of an argument between parties. Ayres works through the terminology plainly, treating burden of proof as a structural device that determines how legal arguments are built and attacked rather than an abstract rule. Short and modular, the lecture is designed to be watched on its own even outside the full course, and it lays out the mechanics any law student needs before tackling actual case arguments.