I do the same as Sonny1318 and have no problems, I have a small plastic tub I line only with a shop towel then put the snake in then the mouse. Then while he/she is working on getting it down I can spot clean their tub/tank, refill water, etc. After I know the mouse has been downed I just pick the snake up and gently put them back in their tub. They almost immediately go right for the hot side hide box to digest for a day or two. The main reason I do it this way is to spot clean the tub, the secondary reason is I've seen a lot of threads regarding snakes with substrate stuck in their mouth's. I don't believe there is anything to the whole aggression thing, I think that depends on the snake itself.