If you think about it, we measure almost everything: time, distance, size, weight, volume, etc. Every THING can be measured. The question is, what unit of measurement is used and how is that unit constructed, compared, and converted? This last part is what we explore in these tutorials.
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Wait, I'm in Europe and my car only tells my distance traveled in kilometers! But I'm used to a units of distance devised by the Romans to measure the average length of 1000 paces of a soldier (the "mile")! How do I operate? These tutorials are about the fundamental skill of unit conversion. Sal's cousin Nadia being a bit confused with this was actually the reason why he started tutoring her (which led to the creation of the Khan Academy). 

Volume measures how much 3-dimensional "space" an object takes up. We'll see in this tutorial that it is an extension of length (1-D) or area (2-D) to three dimensions!