A few popular riddles for adults include, "Is an older $100 bill worth more than a newer one?" The answer is, "Yes. A $100 bill is worth more than a $1 bill." Another example is, "When is homework not homework?" The answer is, "When it's turned in to the teacher."
The best riddles for adults should cause the guesser to think as well as laugh, GoodRiddlesNow.com notes. To add dimension to a riddle, the brain teaser should employ irony, humor and intriguing word play. The site also recommends refraining from using dirty or inappropriate riddles, even if children are not present.
Another popular riddle for adults asks, "Brothers and sisters I have none, but this man's father is my father's son. Who is the man?" The answer is simple: the man is my son. Another example is, "What travels around the world while staying in one corner?" The answer is, "A stamp."
Many popular riddles require logic rather than tricky language. For example, one riddle reads, "Who makes it has no need for it. Who buys it has no use for it. Who uses it can neither see it nor feel it. What is it?" The answer is, "A coffin." BrainDen.com lists this riddle as an "oldie but goodie."