It's about as heavy as a Mouse.
(a.k.a. "House mouse", Mus musculus) (adult)
The weight of a mouse is typically an average of 12 to 30 g. While most mice typically live two to three years, the Methuselah Mouse Prize — an award administered by a non-profit organization which advances aging-related research — was once awarded to a team of scientists whose research resulted in a mouse living just under five years.
It's about nine-tenths as heavy as a Battery (AA).
In other words, 21 grams is 0.880 times the weight of a Battery (AA), and the weight of a Battery (AA) is 1.10 times that amount.(Alkaline)
An "AA" size battery weighs 24 g. AA batteries came into common use during World War II and were given their name by the American National Standards Institute (ANSI) Committee C18 because they were smaller than the "A" size batteries which were invented first.
It's about one-and-two-fifths times as heavy as a Compact disc.
It's about three-fifths as heavy as a Light Bulb.
In other words, 21 grams is 0.620 times the weight of a Light Bulb, and the weight of a Light Bulb is 1.60 times that amount.(a.k.a. lamp) (for A19-size, 60w, incandescent bulb by Bulbrite; package weight)
A standard, sixty-watt light bulb weighs about 34 grams. The tightly-coiled filament inside a bulb of this size would measure about 579mm if uncoiled and stretched out.
It's about one-and-four-fifths times as heavy as a Battery (AAA).
In other words, the weight of a Battery (AAA) is 0.560 times 21 grams.(Alkaline)
An "AAA" size battery weighs 12 g. Batteries are named by the American National Standards Institute (ANSI) Committee C18 and AAA batteries were given their name because they were smaller than the "AA" and "A" size batteries which were invented first.
It's about half as heavy as a Golf Ball.
In other words, 21 grams is 0.45720 times the weight of a Golf Ball, and the weight of a Golf Ball is 2.1870 times that amount.(per R&A USGA Rules of Golf; maximum rule weight)
According to the Rules of Golf (as approved by the United States Golf Association and the Rules Committee of the Royal and Ancient Golf Club of Saint Andrews), golf balls must weigh no more than 45.93 g. Golf balls are not required to have the familiar dimpled pattern, but the design has been popular since its invention in 1905 because it reduces drag while increasing lift.
It's about one-third as heavy as a Tennis Ball.
In other words, 21 grams is 0.3640 times the weight of a Tennis Ball, and the weight of a Tennis Ball is 2.750 times that amount.(ITF Standard; for Type 2 ball)
Per International Tennis Federation standard, regulation tennis balls must weigh about 56.0 g to 59.4 g. Competition balls are also tested for bounce height, with the standards requiring that a ball dropped from 254 cm must bounce back to a height of 135 cm to 147 cm.