I would guess that, even a decade ago, the phrase "mathematical recreation" would have been considered a contradiction in terms. Now, in the age of compulsive Sudoku puzzlers, and an increasing canon of popular mathematics books, this descriptor has become credible.
Symmetry abounds: the wallpaper, your chair, even your own body. Familiar types of symmetry include reflection in a line and rotation about a point. Creating a repeating pattern by translating a core segment to a new place, common in wallpaper, also counts as a symmetry, as does switching without the use of a mirror from an anticlockwise segment to one otherwise identical but oriented clockwise.
We've all been there. You're in a bar with a group of friends. The night draws in. The empties pile up. The conversation turns to sublime speculation and ridiculous argument. How many golf balls would you need to circle the Earth? What's the risk of being killed by a shark? How efficient is wind power? How far does your average Premiership footballer run in a game? How can we put an end to all these questions and go home?