Hi there, I have recently observed a strange effect regarding a fan and was wondering if someone could explain what's going on. I have a 22cm computer case fan which I am using in a project. For my purposes, I had to partially tape over the back of the fan. I also put a large plastic jar lid on the back with a hole on the side in which I instered a long plastic tube. Essentially, the entire back of the fan is blocked except for the space created within the plastic jar lid (which is about 2cm in height and 15 cm in diameter). I had thought this would be enough to create a kind of vacuum in the tube. Well, when I turned the fan on, I did observe a vacuum effect, but not much in the tube.... The front of the fan suddenly started sucking things toward it! I tested this, once I took the jar lid off the back, the fan returned to blowing things away, but as soon as I put the lid back, it started sucking things toward it. And, disappointingly, the vacuum effect in the tube is minimal at best. Can anyone explain what's going on here? Thanks!