It's about 3,000 times as heavy as a Hamster.
In other words, the weight of a Hamster is 0.0003 times 521 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 3,000 times as heavy as a Billiard Ball.
In other words, 521 kilograms is 3,196.12768273590 times the weight of a Billiard Ball, and the weight of a Billiard Ball is 0.00031287861414347 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 3,000 times as heavy as a Puck (Hockey).
In other words, 521 kilograms is 3,196.12768273590 times the weight of a Puck (Hockey), and the weight of a Puck (Hockey) is 0.00031287861414347 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 3,500 times as heavy as a Baseball.
In other words, 521 kilograms is 3,585.89935136220 times the weight of a Baseball, and the weight of a Baseball is 0.00027887006912788 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 4,000 times as heavy as a Battery (D).
In other words, 521 kilograms is 3,860 times the weight of a Battery (D), and the weight of a Battery (D) is 0.000259 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.
It's about 5,500 times as heavy as a Deck of Cards.
In other words, the weight of a Deck of Cards is 0.00018 times 521 kilograms.(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 8,000 times as heavy as a Battery (C).
In other words, the weight of a Battery (C) is 0.00013 times 521 kilograms.(Alkaline)
A "C" size battery weighs 0.065 kilograms. C batteries are typically used in medium-drain devices such as musical instruments.