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Petar Mares
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Estimating the weight of different objects can be successfully done if we compare them to an object of the same type, or a similar type made of the same materials.

Milk is an interesting example.

Milk is a liquid and like most everyday liquids it consists almost entirely of water - 87%, the rest is protein, milk fat, lactose, micronutrients like calcium etc. So we will estimate that milk is a little bit heavier than water. One liter of water is exactly one kilogram in weight, that is the density of water - 1 kg/l. So we can say that a liter of milk has slightly the same density (weight per kilogram), maybe a little above but close.

*Image source: https://www.inchcalculator.com/water-weight-calculator/*

If we check it out it turns out one liter of milk is 1 kilogram and 30 grams, so for smaller amounts we can even approximate the weight of milk with a weight of water.

In fact we can approximate the weight of any liquid this way as long as it has similar density as water, we can estimate the density by examining the thickness of the substance though this is not entirely correct but for an estimation it is ok.

So a two liter bottle of juice, for example will most likely weigh around 2 kilograms as juices are almost entirely water.