a difficult problem
a coarse sieve (as for gravel)
pierce with many holes; "The bullets riddled his body"
set a difficult problem or riddle; "riddle me a riddle"
separate with a riddle, as grain from chaff
spread or diffuse through; "An atmosphere of distrust has permeated this administration"; "music penetrated the entire building"; "His campaign was riddled with accusations and personal attacks"
speak in riddles
explain a riddle