JoJo Siwa iTime Kids Smart Watch, 40 mm

40 or forty commonly refers to:

• 40 (number)
• one of the years 40 BC, AD 40, 1940, 2040

40 or forty may also refer to:

Smart may refer to a high level of intelligence or "street smarts".

"Smart" or SMART may also refer to the following.

A watch is a timepiece carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached by a watch strap or other type of bracelet, including metal bands or leather straps. A pocket watch is carried in a pocket, often attached to a chain. A stopwatch is a watch that measures intervals of time.

During most of their history, beginning in the 16th century, watches were mechanical devices, driven by clockwork, powered by winding a mainspring, and keeping time with an oscillating balance wheel. These are called mechanical watches. In the 1960s the electronic quartz watch was invented, powered by a battery and keeping time with a vibrating quartz crystal. By the 1980s it took over most of the watch market, in what was called the quartz revolution (or the quartz crisis in Switzerland, whose renowned watch industry it decimated). In the 2010s, smartwatches emerged, small wrist-worn computers with touchscreens, with functions that go far beyond timekeeping.

Modern watches often display the day, date, month, and year. Mechanical watches may have extra features ("complications") such as moon-phase displays and different types of tourbillon. Quartz watches often include timers, chronographs, and alarm functions. Smartwatches and more complicated electronic watches may even incorporate calculators, GPS and Bluetooth technology or have heart-rate monitoring capabilities, and some use radio clock technology to regularly correct the time.

Most watches used mainly for timekeeping have quartz movements. But expensive collectible watches, valued more for their elaborate craftsmanship, aesthetic appeal, and glamorous design than for timekeeping, often have traditional mechanical movements, despite being less accurate and more expensive than their electronic counterparts. As of 2019, the most expensive watch ever sold at auction was the Patek Philippe Grandmaster Chime for US$31.2 million.