Let S be the set of uninteresting puzzles on our site:
Assume that S is not empty:This means that there are some uninteresting puzzles on our site.This means that S can only be empty!
Then there also exists one least interesting puzzle.
Let P (member of S) be this least interesting puzzle:
Then we would be very interested to know which puzzle P is.This leads to a contradiction!
This makes P a very interesting puzzle!
However, we assumed that P was uninteresting (member of S).
Conclusion: all puzzles on our site are interesting!
Another proof: our awards and visitors' reactions.