Many words are conspicuous for their repetitions of a single letter, like the S in SASSAFRASS, the O in LOCOMOTOR, the D in DIDDLED and the A in ABRACADABRA. A guessing game can be contrived by deleting the dominant letter in each word, leaving AAFRA, LCMTR, ILE and BRCDBR, then posing the problem of restoration of the dominants - perhaps clued by a key word, which would be SODA for the first example.
Rowe, R. Robinson
"A Word Game of Triples,"
Word Ways: Vol. 2
, Article 9.
Available at: http://digitalcommons.butler.edu/wordways/vol2/iss3/9