Woah...

Some explanation by 'shoonya' which I think is pretty good:

Here you go: metronomes (or "pendula") when on table, oscillate with random phases, since that is how they started and they are "uncoupled" (no energy/information flows from one to other so they do not "know" each other.) When they are all together on the cans, notice that the cans themselves oscillate little, providing coupling/information crossover. which forces "synchronization" in periodic systems (discovered by Huygens in 17th century).