In my 'best-selling((Most-given-away would be more accurate)) book n Mathematical Quotations (Where n ~ 100), my favourite joke is:
“Maths is a game played according to certain simple rules with meaningless marks on paper.”
— David Hilbert (1862-1943), infinite hotelier, mathematical revolutionary, native of Königsberg with 23 problems (but a bridge ain’t one).
That last bit of the bio? My best-constructed maths joke ever. Better than 'Here's looking at Euclid', which it turned out I stole from Alex Bellos.
You see, there's a song by the popular musician J-Zed (or something like that) called '99 Problems'. Apparently, none of these related to female dogs, which is an unusual topic to highlight, but we'll let it slide. With so many problems, Mr Zed must have a lot on his mind.
More interestingly, David Hilbert. Mathematician. One of the most influential mathematicians of the early 20th century, his interests ranged widely; at a conference in Paris in 1900, he outlined 23 then 'open((Meaning they hadn't been proved or disproved)) ' problems in maths, laying down a challenge to the researchers of the day to solve them. Several of them have been proved to be undecidable - an option that didn't even exist in 1900 as Gödel was still some decades away.
Hilbert was born in Königsberg, modern-day Kaliningrad, which is famous in its own right for a problem solved by Euler 150 years or so earlier - probably starting the field of topology. My field, yay! The Königsberg bridges connected islands in the Pregel river and locals amused themselves by trying to walk across each bridge once. Sadly, it can't be done (because of topology, the pinnacle of science) - however, the problem is well and truly closed, and not one of Hilbert's 23.
So, there you go: Hilbert had 23 problems, but a bridge ain't one.