What was ENIAC, and how powerful was the first general-purpose computer?

The answer

ENIAC (Electronic Numerical Integrator and Computer), completed in 1945 at the University of Pennsylvania, was the first general-purpose electronic digital computer. It weighed 30 tons, occupied 1,800 square feet, used 18,000 vacuum tubes, and consumed 150 kilowatts of power. Despite this, it could perform about 5,000 additions per second - impressive for its time, but roughly a billionth of what a modern smartphone can do. Programming it required physically rewiring cables and flipping switches, a process that could take days for a single program.