Ann and Kelly are standing on a river bank, wondering how wide the river is.
Ann is wearing a baseall cap, so she comes up with the following idea: She lowers her cap until she sees the tip of the visor just at the opposite bank of the river.
She then turns around 180 degrees, to face away from the river, being careful not to tilt her head or cap, and has Kelly walk to the spot where she can just see Kelly's shoes.
By pacing off the distance between them, Ann and Kelly figure that Kelly was 50 feet away from Ann. If the ground around the river is level, what, if anything, can Ann and Kelly conclude about how wide the river is? Relate this to triangle congruence.