Lord Kelvin is considered one of the most important physicists of the nineteenth century. With multiple talents and a public man, he is the author of famous aphorisms, but many phrases attributed to him are not more than urban legends. One of the most known is that "there are only two little clouds in the sky of physics" and many variations, alluding the belief of completeness in physics at the beginning of the last century. The "two little clouds" are nothing else than the theory of relativity and quantum mechanics, the two pillars of modern physics. In opposition to what is often propagated, Lord Kelvin seemed to be quite aware of the dead end of classical physics at that time and gave a clear indication to the newcomers of the paths to be followed.
history of physics; classical physics; fundamentals of modern physics