Forty years ago, on April 3rd, 1973, the first ever call on a mobile phone was made. The call was made by its inventor, Martin Cooper, on a device that was ten inches (25.4 cm) long, and weighed in at two and a half pounds (1.13 kilos). That's larger than some laptop computers these days. Today, mobile phones typically weigh in at between four and six ounces.

"The whole idea of a phone call changed," 84-year old Cooper said. "It used to be that when you were making a phone call you were calling a place. Now you're calling a person."

Early models cost about 4,000 USD and took 10 hours to charge. And as future models get more and more hi-tech, its inventor says people should remember one: "Always put an on/off button on the phone," Cooper said. "The phone is supposed to be your slave; you're not supposed to be the slave of your phone."

The United Nations estimates there are some six billion mobile phone subscribers worldwide.

Presented by Adam Justice