‘Obvious’ is the most dangerous word in mathematics. [E. T. Bell]
Keeping in mind the quote above, one should always remember that (especially in Mathematics) that something that appears obvious or even trivial may not be so easy after all. In this talk, the speaker gives an idea of why this is true, leading a tour through the concept of cardinality, sets and the well-known rational and real numbers, which turn out to be harbouring some hidden secrets.