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The can opener.
we think no
his hardest invention was the telephone
The Invention of the Telegraph by Joseph Henry in 1831 led to the invention of the phone.
CD's
A tin can opener.
The hydrophone :)
The tin can opener was invented in 1858. This was necessary after the invention of the tin can 48 years earlier, in 1810.
The Walkman led to the invention of the iPod.
The steam engine
The can opener was invented after the invention of the can. Before it's arrival, cans were opened by many and dangerous ways. The dedicated opener made the operation quicker and safer than previous methods.
A caveman.