from NFL.COM:
Touchdown: When any part of the ball, legally in possession of a player inbounds, breaks the plane of the opponent?s goal line, provided it is not a touchback.
Possession: When a player controls the ball throughout the act of clearly touching both feet, or any other part of his body other than his hand(s), to the ground inbounds.
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The receiver established possession since he established control of the ball throughout the act of him catching it and 2 feet touching the ground. The ball was in possession by the player in the endzone = Touchdown.
Touchdown: When any part of the ball, legally in possession of a player inbounds, breaks the plane of the opponent?s goal line, provided it is not a touchback.
Possession: When a player controls the ball throughout the act of clearly touching both feet, or any other part of his body other than his hand(s), to the ground inbounds.
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The receiver established possession since he established control of the ball throughout the act of him catching it and 2 feet touching the ground. The ball was in possession by the player in the endzone = Touchdown.

