conundrum, puzzle, enigma, poser, question, mystery, problem, brain-teaser or Brit brain-twister:
The cracker contained a whistle, paper hat, and the inevitable riddle.
1 perforate, pepper, puncture, pierce, honeycomb:
The back of the bookcase was riddled with tiny worm-holes.
2 penetrate, infest, infect, pervade, permeate, fill, spread:
An investigation revealed that the entire department was riddled with corruption.
3 sieve, colander or cullender, strainer, grating, screen, sifter, filter:
He shovelled the dirt through the riddle to sift out the larger stones.