The mistake in Lenny's proof lies in the reasoning regarding the angles and which triangles are being considered. Specifically, he incorrectly identifies the angles involved in the right angle definition of a perpendicular bisector.
The correct angles in the context should be ∠LMI and ∠LMJ, not ∠MLI and ∠MLJ. Thus, the proper angle notation must be applied to emphasize the angles at point L with respect to the segments from the midpoint to the endpoints of segment IJ.
Therefore, the correct response is: The definition of a perpendicular bisector tells you that ∠LMI and ∠LMJ are right angles, not that ∠MLI and ∠MLJ are right angles.