You just never know what it's going to be like in a building. Even one you have been in before. I went to one building that is always so hot in the winter that you have to open all of the windows in the classrooms, if you can. I dress accordingly, and have even brought a fan with me in December! However, I was in this building the other day and apparently the one classroom I was teaching in did not have the heat problem. I didn't bring a sweater, and was wearing a short sleeve dress, and I was freezing all day long. It's taught me to always dress in layers, even if you know the situarion. I usually keep an extra set of clothes in the car, just in case, and that can come in handy too. If I remember it's there!