Invention often comes in stages and phases, when existing technology is looked at and mixed with new creative insights, a process that sometimes takes thousands of years.

This makes it hard to pinpoint the exact time and place a certain object is invented.

• The wheel and the axle didn't suddenly spring up, but were a gradual, phased invention, starting with rollers that reduced friction on the ground, and helped the Egyptians and Mesopotamians move heavy objects.
• The oldest discovered axle was found inside a potter’s wheel in...