Just over one hundred and fifty years ago, Maxwell first posed the thought experiment that became known as â€œMaxwellâ€™s demon.â€ Designed to understand more deeply the nature of the newly formulated second law of thermodynamics, the demon was to play a long, controversial role in the development of statistical physics. Just two months later, Maxwellâ€™s paper â€œOn governorsâ€ gave the first analysis of a feedback system. These two foundational works reflect the fundamental and practical aspects of control. I will present an experiment that unites the two: using feedback to create â€œimpossibleâ€ dynamics, we make a Maxwell demon that can reach the fundamental limits to control set by thermodynamics. We testâ€”and then extendâ€”Rolf Landauerâ€™s 1961 prediction that information erasure requires at least as much work as can be extracted from a system by virtue of information. We use this identity to infer the form of the two-state nonequilibrium entropy function (Gibbs-Shannon entropy). These fundamental thermodynamic tests are benchmarks for evaluating the performance of practical information engines, such as those active within cells and other complex systems.