It's about as heavy as a Deck of Cards
It's about seven-tenths as heavy as a Battery (D)
In other words, 0.210 pounds is 0.7060 times the weight of a Battery (D), and the weight of a Battery (D) is 1.420 times that amount.(Alkaline)
A "D" size battery weighs 0.2980 pounds. The D size battery is the oldest of the modern battery sizes, first invented by the National Carbon Company in 1898.
It's about one-and-a-half times as heavy as a Battery (C)
In other words, the weight of a Battery (C) is 0.670 times 0.210 pounds.(Alkaline)
A "C" size battery weighs 0.140 pounds. C batteries are typically used in medium-drain devices such as musical instruments.
It's about two-thirds as heavy as a Baseball
In other words, 0.210 pounds is 0.655609756097560 times the weight of a Baseball, and the weight of a Baseball is 1.52529761904760 times that amount.(MLB rules)
Per Major League Baseball rules, a baseball must weigh between 0.31250 pounds and 0.3281250 pounds. 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-and-two-thirds times as heavy as a Tennis Ball
In other words, the weight of a Tennis Ball is 0.6060 times 0.210 pounds.(ITF Standard; for Type 2 ball)
Per International Tennis Federation standard, regulation tennis balls must weigh about 0.1270 pounds. 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.
It's about three-fifths as heavy as a Billiard Ball
In other words, 0.210 pounds is 0.584347826086960 times the weight of a Billiard Ball, and the weight of a Billiard Ball is 1.71130952380950 times that amount.(American-style, WPA specifications)
Per the World Pool-Billiard Association, a billiard ball weight must weigh between 0.343750 pounds and 0.3750 pounds. 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 three-fifths as heavy as a Puck (Hockey)
In other words, 0.210 pounds is 0.584347826086960 times the weight of a Puck (Hockey), and the weight of a Puck (Hockey) is 1.71130952380950 times that amount.(Ice hockey)
A standard ice hockey puck weighs between 0.343750 pounds and 0.3750 pounds. Hockey pucks can be shot or passed at speeds of up to 160 kph (99 mph) during games.