Why bending a metal makes it both stronger — and more brittle — at the same time.
Bend a paperclip back and forth a few times and it gets harder to bend — right before it snaps. That's not a coincidence and it's not two unrelated things happening at once. Both effects trace back to the same microscopic actor: dislocations, the line defects that actually carry plastic deformation through a metal's crystal lattice. Understanding what blocks dislocation motion — and what happens when a metal is forced to generate far more of them than it started with — explains why smaller grains make metals stronger, and why cold-working a part is a strength-for-ductility trade you can't avoid, only manage.