1. Young man, in mathematics you don't understand things
Young man, in mathematics you don't understand things. You just get used to them.
— John von Neumann

2. Orders of Infinity
If each element in red set can be paired (1-to-1) with an element in the blue set, sets have same cardinality.
Smallest infinite set is the set of natural numbers N = {1,2,3…}, said to be a countable infinity, cardinal number 0.
Countable sets: N, Z, Q
Next cardinal number is c, the cardinality of the continuum.
Sets with cardinality c: [0,1], R, C, Rn
Lines, surfaces, volumes,… have same “number” of points!