Do you still remember IBM Simon? Or are you old enough to ever have an opportunity to meet it? But I believe you must have known or had an iPhone. Actually, IBM Simon is the "father" of smart phones. It came out around 1995 and was the most creative electronic product at that time. However, it didn't look like what a smart phone looks at present and it weighed 510g with a thickness of 38mm. Can you believe it? It's totally impossible for you to put it in your pocket.
With the improvement of functions and people's aesthetic attitude towards cell phones, smart phones have experienced a series of changes in terms of size and figure. Figure 1 indicates the thickness change of smart phones in the past years.