Why did Aaron and his sons lay their hands on the bull?
Leviticus 8:14 (ESV)
14Then he brought the bull of the sin offering, and Aaron and his sons laid their hands on the head of the bull of the sin offering.
The first thing that the priests did when sacrificing the bull as sin offering was laying their hands on the animal. This symbolized the transference of the sin from the priests to the animal.
14 Then he brought the bull of the sin offering, and Aaron and his sons laid their hands on the head of the bull of the sin offering.