English "riddle" is from Middle English ridelle "coarse sieve," from late Old English hriddel "sieve," altered by dissimilation from Old English hridder "sieve" 
A riddle may be square, rectangular or circular in shape, with a rim made from wood, metal, plastic or beechwood, holding in place a steel wire mesh that may either be handwoven or machine-made. A typical circular 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.