here is another seemingly wrong thing - most metals get brittle when they harden. or in the meme-y way

when the metal hardens, it breaks more easily, man…

Those who want the reason, when metals are hardened (by means by annealing and subsequent quenching, or cold working, or hot working), multiple “defects” (point, line, planar as well)(in this context, these words have specific meaning, but that is not very important). these “defects” resist motion, so if you apply some amount of force, they do not deform as much (not the technically correct wording, but close enough) - this is hardness. but simultaneously, when these defects form, they also act as psitions of “stress concentration” (imagine weak points). so more there are defects, harder they become, simultaneously, more likely to break (the likeliness to break is quantised by elongation at fracture, hardness has multiple quantisations, but we usually do some correlation-ary things (like measuring the area/depth of deformation made by some specific method))

One comma away from an entirely different interpretation.