Damage to cogs and chainrings happens when the chain “stretches.” I’ve put that in quotes, because the plates that hold the chain together don’t actually stretch (none of us weighs enough to stress the steel chain plates beyond their yield point); what happens is that when the rollers wear on the inside, a gap opens between the rollers and the pins, and this gap allows the chain pins to pull farther apart than on a new chain. This causes an apparent elongation of the chain that accelerates wear on the cogs and chainrings, because the chain no longer meshes evenly with all the gear teeth (thus spreading the wear evenly among many teeth); instead, the pedaling force becomes concentrated on only the first few links of the chain that contact the gears. This greatly increases the contact pressure between the chain and gears which, when combined with the aforementioned grinding paste effect, rapidly wears out the gears. The cogs, having fewer teeth than the chainrings, wear out first, but the chainrings will eventually wear out, too.

To minimize wear on the gears, it’s prudent to not only clean the chain frequently, but to also replace the chain when it “stretches” too much. Park Tool makes a $10 gauge (the CC-3 Chain Checker) that measures chain wear and will indicate when the chain needs replacing. It’s a worthwhile tool to own, because quality chains cost more than $10, so the gauge helps avoid replacing chains too often.

When the end of the world does come, will there still be a Heaven?

I’m disappointed that they didn’t do a better job, but it’s certainly not the end of the world.