I've had 2 male mice named Shrimp and Prawn for 5 weeks now. When I give them treats in my hand, one of them would often bite my finger tips hard. Usually, it hurts but doesn't draw blood but I've had a past situation where they did. I had just returned from a 2 night trip, they had plenty of food and water, and I went to give them some sunflower seeds. They ate a few seeds, and then one of them latched onto my finger and did not let me pull away and drew blood. Why do they do this? Right before the trip, I believed they were past the biting. It is quite painful even after the bleeding has stopped.