Question: Special birthdays such as the first, 21st (adulthood), and 100th (centenarian) are common, but around the world, what are some uncommon special ones?

Answer: Japan: 60, 69, 76, 77, 87, 89, 98 are traditional celebrations of longevity ("chojuiwai"). Ages 3 and 5 for boys and 3 and 7 for girls are also considered significant. (University of Hawaii's Center for Japanese Studies)

In China, 30 signals adulthood. And in several Latino cultures, a girl's 15th birthday (a "Quinceanera") marks this passage.

Many African cultures have coming-of-age parties in groups rather than for individual children, much like certain Lutheran or Catholic confirmation ceremonies.

Northern Germany: A man still without a wife on his 30th birthday sweeps the stairs of city hall to show he's available and not a bad bargain since he cleans house well. An unmarried woman polishes her front door knob.

Northern Europe: Birthdays 10, 20, 30, 40, etc., are much-gifted "round" years, says University of California-Los Angeles folklorist Timothy R. Tangherlini.

Vietnam: Tet, the start of the lunar New Year, is considered everyone's birthday. Babies turn 1 on Tet no matter the exact day they were born.

Korean babies are "1" at birth and turn a year older on lunar New Year's, adds Tangherlini. "Tol" is celebration of the first anniversary of the birth, so a child born right before lunar New Year might be considered "2 years old" from day 3 through the next New Year. Sixty is most significant, marking a full zodiacal cycle and a full lifetime — "anything after that is icing on the cake."

Question: On which day of the year does a house cast its longest shadow, and why might some folks want to know?

Answer: Sunshine access is the issue. Shadows are longest on the shortest day of the year, which is the winter solstice (in the northern hemisphere about Dec. 22), say Jeffrey Bennett and William Briggs in "Using and Understanding Mathematics." Some cities have laws prohibiting property owners from constructing new houses or additions that cast shadows on nearby houses, thereby "allowing everyone access to the sun for the use of solar energy devices." One such law prohibits structures casting a noontime shadow reaching farther than would be cast by a 12-foot fence on the property line, on the shortest day of the year. Hmmm — always wondered when those trigonometry tables would come in handy.

Send STRANGE questions to brothers Bill and Rich at [email protected], coauthors of "Can a Guy Get Pregnant? Scientific Answers to Everyday (and Not-So- Everyday) Questions," from Pi Press.