How (if at all) do you know that you are not a brain in a vat?

Our whole life (attitudes, reactions, actions) is based on knowledge. Depending on our present state of knowledge, we are going to react to different situations in different ways, give different answers to different questions. To do that however, one has to know something. Even though we do not tend to question our knowledge in everyday life, it is interesting to do so in order to (ideally) find an answer to the sceptics who deny that knowledge exists, who assert that we cannot know anything for sure.