See also Winnowing
A riddle is a large sieve used to separate soil or compost particles, or for separating soil from vegetables.
A riddle is often circular in shape, with a rim made from beechwood, holding in place a steel wire mesh that may either be hand woven or machine made. A typical riddle is approximately 18" in diameter and the mesh may have a spacing of something like 1 1/2", 1", 5/8 ", 1/2" or 3/8".
A riddle is typically used to improve soil quality by allowing the gardener to sieve through soil and remove stones, twigs, large lumps of clay etc and hence provide a finer tilth. Smaller riddles can be used to separate soil very finely for seeds and early potting. Riddles may also be used to help remove soil from harvested vegetables.
English "riddle" is from from Middle English ridelle "coarse sieve," from late Old English hriddel "sieve," altered by dissimilation from Old English hridder "sieve"