n.
a difficult problem
同义词：conundrumenigmabrain-teaser
a coarse sieve (as for gravel)
Riddles of Natural Categorization [and Discussion]
An implemented model of punning riddles
Good to Eat: Riddles of Food and Culture. by Marvin Harris
Reservoir riddles: suggestions for echo state network research
Reservoir riddles: suggestions for echo state network research
Comment on Vayda's review of good to eat: Riddles of food and culture
Hybridity, So What? The Anti-Hybridity Backlash and the Riddles of Recognition
Cerebral malaria -- a neurovascular pathology with many riddles still to be solved.
Benthic invertebrate production—facilitating answers to ecological riddles in freshwater ecosystems
BioOne Online Journals - Benthic invertebrate production—facilitating answers to ecological riddles in freshwater ecosystems
