The tin can was patented in 1810, and for most of the next half century the instructions amounted to: find a hammer. One early label told you to cut round the top near the outer edge with a chisel and hammer; soldiers used bayonets. The first openers made for the job were patented in the 1850s, and the American one, Ezra Warner’s of 1858, was a curved blade you stabbed in and sawed round with, good enough for the army and too dangerous for a kitchen. It’s tempting to read that gap as nobody thinking of it. More likely the can had to change first: the early ones were heavy iron, made to survive a ship’s hold, and nothing much gentler than a chisel was going to get through. Some inventions wait for an idea. This one waited for the problem to get thinner.