Dislocations in the crystal structure. Here's a demonstration using a raft of small bubbles to simulate atoms in a metal (actual demonstration starts at 5:12 if

Dislocations in the crystal structure. Here's a demonstration using a raft of small bubbles to simulate atoms in a metal (actual demonstration starts at 5:12 if you want to skip the introduction): https://www.youtube.com/watch?v=sn1Y6zIS91g https://www.youtube.com/watch?v=sn1Y6zIS91g You can see the same grains forming as the bubble raft is deformed.