It's about one-tenth as heavy as a Baseball
In other words, 0.021 kilograms is 0.144537209939740 times the weight of a Baseball, and the weight of a Baseball is 6.91863361979190 times that amount.(MLB rules)
Per Major League Baseball rules, a baseball must weigh between 0.1417476156250 kilograms and 0.148834996406250 kilograms. A major league pitcher can throw a baseball at up to 150 kph (91 mph) (with a fastball pitch), with record-setting pitches as fast as 171 kph (106 mph).
It's about one-tenth as heavy as a Billiard Ball
In other words, 0.021 kilograms is 0.128826643641950 times the weight of a Billiard Ball, and the weight of a Billiard Ball is 7.76236942708310 times that amount.(American-style, WPA specifications)
Per the World Pool-Billiard Association, a billiard ball weight must weigh between 0.15592237718750 kilograms and 0.170097138750 kilograms. According to legend, the difficulty in obtaining elephant ivory for the manufacture of billiard balls in the 19th century inspired the search for a replacement material, which led to the development of the first industrial plastic.
It's about one-tenth as heavy as a Puck (Hockey)
In other words, 0.021 kilograms is 0.128826643641950 times the weight of a Puck (Hockey), and the weight of a Puck (Hockey) is 7.76236942708310 times that amount.(Ice hockey)
A standard ice hockey puck weighs between 0.15592237718750 kilograms and 0.170097138750 kilograms. Hockey pucks can be shot or passed at speeds of up to 160 kph (99 mph) during games.
It's about eight-and-a-half times as heavy as a Penny
In other words, the weight of a Penny is 0.120 times 0.021 kilograms.(United States) (formally one-cent coin) (Union shield design, 2010-present)
The weight of a United States penny is 0.0025 kilograms. According to a 2010 report by the United States Mint, the cost of manufacturing and distributing a penny is $0.0179 — more than its face-value.
It's about one-tenth as heavy as a Hamster
In other words, the weight of a Hamster is 10 times 0.021 kilograms.(for Syrian hamster, a.k.a. golden hamster, a.k.a. Mesocricetus auratus; adult)
An adult Syrian hamster has a typical weight of about 0.20 kilograms. Known for transporting food in the pouches of their cheeks so that they can bury it later, some hamsters can store up to 25 kg of food in their borrows.
It's about twelve times as heavy as a Playing Card
In other words, the weight of a Playing Card is 0.083 times 0.021 kilograms.(ISO 216 B8 size, 2.5" x 3.5")
A single ISO 216 B8-size playing card weighs 0.0018 kilograms. Although this size is the most common home playing card, many casinos use "bridge size" cards — 2,640 sq. mm (4.036 sq. in) smaller in area.