A theorem is a statement proved from definitions, postulates and earlier theorems, one justified step at a time. This week the crew proves that vertical angles are congruent, that a transversal across parallel lines makes congruent alternate interior and corresponding angles, that the points on a perpendicular bisector are exactly the points equally far from both ends, that a triangle's angles add to 180°, that an isosceles triangle's base angles match, that a midsegment is parallel to the third side and half as long, and that a parallelogram's opposite sides and angles are congruent while its diagonals bisect each other. The frame on the sawhorses puts every proof to work.