It's about three times as heavy as an Eye (Human)
It's about one-third as heavy as a Battery (C)
In other words, the weight of a Battery (C) is 3.10 times 0.021 kilograms.(Alkaline)
A "C" size battery weighs 0.065 kilograms. C batteries are typically used in medium-drain devices such as musical instruments.
It's about three-and-a-half times as heavy as a Quarter
In other words, 0.021 kilograms is 3.7040 times the weight of a Quarter, and the weight of a Quarter is 0.270 times that amount.(United States quarter-dollar coin) (a.k.a. Quarter dollar, a.k.a. America the Beautiful quarter, a.k.a. two bits)
The America the Beautiful series quarters, which began to enter circulation in 2017, weigh 0.00567 kilograms each. In answer to the common riddle, there are 119 ridges around the edge of the coin in its current design.
It's about four times as heavy as a sheet of Paper
In other words, 0.021 kilograms is 4.20 times the weight of a sheet of Paper, and the weight of a sheet of Paper is 0.240 times that amount.(for US Letter, a.k.a. ANSI A; 215.9mm by 279.4mm (8.5 in x 11 in); 20lb)
The weight of a ream (500 sheets) of 9.10 kilograms, 8.5 inch x 11 inch paper is 2 kg, each sheet weighing 0.005 kilograms. Paper of this size has a thickness of 0.004 caliper (0.1 mm).
It's about one-fifth as heavy as a Deck of Cards
In other words, 0.021 kilograms is 0.220 times the weight of a Deck of Cards, and the weight of a Deck of Cards is 4.50 times that amount.(ISO 216 B8 size, a.k.a. "poker size" 63 x 88 mm, 52-card deck)
A deck of 52 ISO 216 B8-size playing cards weighs 0.094 kilograms. Utilizing nearly 3,700 decks, the record-holding largest card tower was built in early 2010, stood over 3.5 m (11 ft), and weighed over 272 kg.
It's about five times as heavy as a Die (Dice)
In other words, 0.021 kilograms is 5.10 times the weight of a Die (Dice), and the weight of a Die (Dice) is 0.20 times that amount.(for six-sided, 16 mm, rounded cube)
A die, of the size most commonly used in board games and casinos, typically weight 0.0041 kilograms. The use of dice and dice-like tools predates written history, with the oldest known examples belonging to a 5,000-year old backgammon set uncovered during an archaeological dig in modern-day Iran.
It's about one-fifth as heavy as a Battery (D)
In other words, 0.021 kilograms is 0.1560 times the weight of a Battery (D), and the weight of a Battery (D) is 6.410 times that amount.(Alkaline)
A "D" size battery weighs 0.1350 kilograms. The D size battery is the oldest of the modern battery sizes, first invented by the National Carbon Company in 1898.