Things fall down, not up. Weddings have a bride and a groom (sometimes). If people yell at you, they’re probably angry. One of the reasons that computers seem dumber than humans is that they don’t have common sense — a myriad of simple facts about everyday life and the ability to make use of that knowledge easily when appropriate. A long-standing dream of artificial intelligence has been to put that kind of knowledge into computers, but it has proven slow and difficult. But considerable progress has been made over the last few years. There are now large knowledge bases of common sense knowledge and better ways of using it then we have had before. We may have gotten too used to putting common sense in that category of impossible problems and overlooked opportunities to actually put this kind of knowledge to work. We need to explore new interface designs that don’t require complete solutions to the common sense problem, but can make good use of partial knowledge and human-computer collaboration. As the complexity of computer applications grows, it may be that the only way to make applications more helpful and avoid stupid mistakes and annoying interruptions is to make use of common sense knowledge. Cell phones should know enough not to ring during the concert. Calendars should warn you if you try to schedule a meeting at 2 AM or plan to take a vegetarian to a steak house. Cameras should realize that if you took a group of pictures within a span of two hours, they are probably all of the same event. This talk will explore the state of the art in common sense knowledge and reflect on design principles for building interfaces that can exploit this knowledge to make more usable and helpful interfaces.