Imagine walking down a busy sidewalk and seeing someone talk into a box with no cord attached. On April 3, 1973, Martin Cooper stepped onto a sidewalk in New York City carrying something nobody had ever seen. A phone with no cord. It weighed about 2.5 pounds -- as heavy as a small brick. Cooper worked for a company called Motorola. He pressed the buttons and made the first portable cell phone call in history. And he knew exactly who to call. He dialed his rival, Joel Engel, who worked at a competing company called Bell Labs. Both companies had been racing to build the first wireless phone. Cooper wanted Engel to know that Motorola had won. The phone Cooper used was called the DynaTAC. Its battery lasted only about 20 minutes, and the phone took 10 hours to charge. It could not send text messages, take pictures, or connect to the internet -- those features were decades away. But it proved that people could make calls while walking down the street. It took another 10 years before the first commercial cell phone went on sale in 1983. It cost $3,995 -- about $12,000 in today's money. Few people could afford one. But the technology improved quickly. Phones got smaller, cheaper, and more powerful. Today, over five billion people worldwide own a mobile phone. Most of those phones are more powerful than the computers that sent astronauts to the Moon. Every one of them traces back to that moment on a New York sidewalk.