I was going through vector and scalar quantities (the way they are taught in high school), and this is how I think students are supposed to understand it: Scalar quantities are quantities that add like numbers. For e.g. Mass. If I add 100 g of water to a bucket and then add a further 100 g, I could only get 200 g of water as a resultant. Vector quantities are quantities with magnitude and direction that add according to the triangle rule. If a person travels 5 m in one direction and another 5 m in some other direction, the resultant displacement need not be 10 m and depends on the direction of the two original displacements. The resultant is given by triangle rule. At this point, one may ask, how is temperature a scalar? Granted, it doesn't have a direction. But how do they add like numbers? If I take some substance at 300 K and mix it with something at 300 K, the resultant temperature is never 600 K. Can someone comment on how to answer such a question? I would argue by the following example: consider a person somewhere in Africa travels 5 m north. Another person in the US travels 5 m East. Now I could treat these two displacements as vectors and add them up to get 5√2 m North-East. But this resultant vector doesn't stand for anything physical. So I guess the question can be asked: what does 'addition of temperatures' mean physically? Also, are my descriptions of vector and scalar quantities good enough?