The question of usefulness of mathematics in everyday life is a cliche, and I am not asking that.
**What are some magic tricks/objects$^*$/algorithms/other curious stuff, which has very deep mathematical principle governing them ?**
Examples: (i) Rubik's cube (Lot's of stuff from group theory and combinatorics) (ii) The trick that you pick two number, and keep adding the largest to the second largest, and tell (after some iterations) only one, but from that you can derive the next one (Which is based on $\lim_{n \rightarrow \text{large numbers}} \frac{F_(n+1)}{F_n} \approx \phi$ )