See a Magnum Photos gallery of boys with sticks.

A few weeks ago, we posted our annual, Christmastime list of the Explainer questions we’d been either unwilling or unable to answer in the previous year. Questions such as, Why aren’t bathtubs bigger? And, If a person is allergic to cats, would he also be allergic to a lion?Then we asked our readers—you—to pick the entry that most deserved an answer. More than 20,000 people cast a vote, and the winning question is presented below. But first, the runners-up:

In third place, with 1,209 votes: Could mankind actually blow up the moon? Blast it with nuclear missiles until it was just rubble?

In second place, with 1,754 votes: Are all languages equally lip-readable?

And in first place, with 2,874 votes, the winner by a landslide and the Explainer’s Question of the Year for 2010:

I’ve always pondered why boys like having sticks. Whether it be walking down a hiking trail with a stick they picked up or running a stick across a white picket fence, boys (including me when I was small) seem to have a knack for having a stick. Is there some kind of explanation for this behavior?

The answer: Yes, of a sort.

Boys may like sticks because they’re so well-suited to boyish behavior like play-fighting—a long, thin piece of wood can serve as either an imaginary sword or gun. (Juveniles in other animal species also play-fight with objects.) But there hasn’t been much empirical work relevant to this question, so we can’t say for sure whether boys are more inclined than girls to play with sticks, or whether boys prefer sticks to other objects.

Here’s what developmental psychologists do know: Boys can be very discriminating when it comes to choosing a plaything. Studies of children’s recreational habits find that when given the choice, boys tend to avoid dolls and other unmanly items, while girls are more flexible—they’ll play with Little Mommy or a toy truck. This “snips-and-snails” effect takes hold from a very early age: One experiment concluded that 3-month-old boy babies spend more time staring at trucks than dolls (and vice-versa for girls); others suggest that sex-based preferences for certain objects take hold between the ages of 1 and 2. Either way, the inclination seems to be in place before children have any understanding of their own gender.

To some extent, babies and toddlers have these preferences for cultural reasons: Parents give their kids gender-appropriate toys, and otherwise show approval for certain kinds of play but not others. It’s altogether possible—almost certain, in fact—that a boy will have internalized these unspoken rules about which objects should be used for fun, and which ones he should avoid.

There’s another theory for the gender preferences , though. Maybe the boys aren’t just following cultural rules when they shun dolls in favor of toy trucks. Perhaps the choice of object has something to do with the sorts of activities that they prefer: If boys like to run around and manipulate things, a plastic car with rotating wheels might seem more interesting than a stuffed animal; likewise, if girls like to snuggle up, the animal would be more interesting. These inclinations could themselves be socially conditioned, of course, but they might also reflect some earlier genetic or hormonal influence.

It turns out that young girls who were exposed in utero to high levels of androgens—a condition called congenital adrenal hyperplasia —end up showing a preference for boyish objects , even when their parents urge them in other directions. And a recent study at the Yerkes Primate Research Center found that juvenile rhesus monkeys show the same sex-related preferences as human children when offered a choice between plastic wagons and Raggedy-Ann dolls. (The boy monkeys played with the wagons; the girls went either way.) That suggests culture isn’t the whole story: To some extent, at least, boys are naturally drawn to certain objects. They do indeed have a “knack” for playing with trucks.

OK, back to sticks. If sticks happen to fall in the same category as “masculine” toys like toy trucks and fake weapons, then there’s good reason to believe that a boy would be drawn to them from early infancy. Which raises the question: Which aspects of a stick—or any other play object—make it more boyish or less girly?

There are plenty of theories. Some have argued that color plays a part—that girls are naturally drawn to pink objects, for example. Or that boys prefer sharp lines, while girls like rounder things. (In art class, boys tend to draw more angular shapes .) Or maybe boys like harder objects, while girls prefer softer ones. Recent research suggests that such tendencies develop relatively late: Among 1- and 2-year-olds, at least, both boys and girls seem to prefer reddish colors and rounded shapes . Less is known about which qualities might be most relevant to infants.

In any case, a look at the animal kingdom reveals that there’s more than one way to play with a stick: A new study , published in December, looked at a wild group of chimpanzees in Uganda and found that the young females were using sticks as dolls . That is to say, they were holding the sticks while they slept and patting them like babies. (The chimps also used sticks to hit each other, and poke into holes looking for water or honey.)

This behavior has never before been seen in the wild and appears to be unusual among non-human primates. (The study’s conclusions were also based on a rather sparse data-set collected over a 14-year period.) Girl rhesus monkeys, for example, don’t bother with pretend-stick-babies: They just kidnap real babies from lower-ranked females in the group and play with those. And primates aren’t the only animals that play with sticks: Birds drop them and then catch them again in midflight ; sardines and needlefish leap over floating twigs for no apparent reason; and both male and female porcupines masturbate using sticks .

Got a question about today’s news? Ask the Explainer .

Explainer thanks Gerianne Alexander of Texas A&M University, Gordon Burghardt of the University of Tennessee, Kim Wallen of Emory University, and Ed Yong of Not Exactly Rocket Science .

Previous questions of the year:

2009 : If a Siamese twin commits murder, does his brother get punished, too?

2008 : What is the most disloyal dog breed?

2007 : Why don’t we drop medical waste and nuclear waste into active volcanoes, the “ultimate high-temperature incinerators”?

2006 : Can a bar of soap get dirty, or is it self-cleaning because it’s soap?

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