Clock arithmetic and modular arithmetic
http://www.shodor.org/interactivate/discussions/morerem.html
This site gives a nice introduction to modular arithmetic.
http://betterexplained.com/articles/fun-with-modular-arithmetic/
Is there really such a thing as “infinity”?
http://www.math.toronto.edu/mathnet/plain/answers/infinity.html… See the whole entry
Infinity (from scratch) – slightly humorous, but mathematically correct. (time: 9:42)
The smallest Infinity (time: 9:33)
… See the whole entry
This site introduces the idea of infinite sets:
https://www.geeksforgeeks.org/infinite-set/
This site introduces the idea that there are different sizes of infinity:
https://www.quantamagazine.org/mathematicians-measure-infinities-find-theyre-equal-20170912/