On April 3, 1973, people were amazed when American engineer Martin Cooper made a phone call from a New York City sidewalk using a cell phone he had developed. It weighed about 0.9 kg (2 lb.) — almost as much as a liter (quart) of milk.
At first, you could only talk on cell phones. Then smartphones were launched in 1993. Now you can take photos, access the Internet, text message, play games, watch movies — the list goes on and on.
Today, there are more than 5 billion cell phones. They have been used to find survivors during natural disasters, provide banking services where banks are scarce and even organize protestors to start revolutions.