It's about one-fiftieth as heavy as a liter of Water
In other words, 0.021 kilograms is 0.02099941 times the weight of a liter of Water, and the weight of a liter of Water is 47.620390 times that amount.(fresh; 4° C / 39.2° F; maximum density)
Contrary to popular belief, 1 liter (0.0.264172052358148 gal) of water at 4° C / 39.2° F (its point of maximum density) weighs 1.000028 kg rather than 1 kg exactly. The total volume of water on the planet Earth is about 13,590,000,000,000,000,000 (13.59 quntillion) L (3,590,000,000,000,000,000 gallons).
It's about one-sixty-fifth as heavy as a Human Brain
In other words, the weight of a Human Brain is 64.10 times 0.021 kilograms.(adult)
The average weight of an adult human brain is between 1.350 kilograms. In humans and primates, sight and vision utilize about 30 different parts of the brain which together account for about half of the brain's size.
It's about 100 times as heavy as a Raindrop
In other words, the weight of a Raindrop is 0.01 times 0.021 kilograms.(a.k.a. drop, a.k.a. droplet) (average)
The weight of a raindrop depends heavily on the type of weather, but averages about 0.0002 kilograms. The familiar teardrop shape of a raindrop is actually an optical illusion, caused by the reflection of the light and the motion of the drop.
It's about one-one-hundredth as heavy as a Chihuahua
In other words, 0.021 kilograms is 0.00933 times the weight of a Chihuahua, and the weight of a Chihuahua is 107 times that amount.(a.k.a. Chihuahue(o) (FCI standard)
The F&d&ration Cynologique Internationale (FCI) calls for Chihuahuas to weigh about 1.5 to 3 kg. Relative to their body sizes, Chihuahuas have the largest brain size of any dog breed.
It's about one-one-hundred-fiftieth as heavy as a Brick
In other words, the weight of a Brick is 140 times 0.021 kilograms.(Standard, traditional face brick - 1,900 cu. cm (120 cu. in))
A standard, traditional face brick weighs an average of 2.9 kg. The steeple of the Church of St. Martin in Landshut, Germany is said to be the tallest brick tower in the world at a height of 130.6 m (428.5 ft) and was constructed using an estimated 650,000 bricks.
It's about one-two-hundredth as heavy as a Cat
In other words, the weight of a Cat is 200 times 0.021 kilograms.(Felis catus) (Domestic; healthy weight)
The average domestic cat should weigh between 4 and 5 kg. Cats are known for their righting reflex, which allows them to land on their feet and survive falls from heights of 1 m (3 ft) up to about 21 m.
It's about one-two-hundred-fiftieth as heavy as a Gallon of Paint