The regular hexagon hidden in a cube unfolds to a straight line on a net of the cube.
Source: https://twitter.com/panlepan/status/1138686590216298497?s=20 by @panlepan
Six Points on a Circle
Take 6 points on a circle such that every second edge (green chords) has length equal to the radius of the circle. Then the midpoints of the other three sides of the cyclic hexagon form an equilateral triangle.
Read more: https://www.archimedes-lab.org/2-Sunday_puzzle_43.html
A curious right triangle
The sides of a pentagon, hexagon, and decagon, inscribed in congruent circles, form a right triangle.