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NPR science correspondent, Robert Krulwich, has been exploring the ends and outs of a more pedestrian kind of movement: walking.

ROBERT KRULWICH: OK, this is a little weird. If I took you to a parking lot, or maybe a field, some wide open space; and I put a blindfold on you and I said to you, here's what I want you to do, just walk forward and try to stay in a straight line. This is what scientist Jan Souman asked his subjects to do in Germany.

Mr. JAN SOUMAN (Scientist): Walk straight for one hour, exactly. So it's a really boring task. You just walk, they walk, they walk the whole time they're walking in a straight line.

KRULWICH: And the thing is, it can't be done. Jan says everybody who tries it fails.

Mr. SOUMAN: Some people say...

KRULWICH: Everyone?

Mr. SOUMAN: Everyone.

KRULWICH: Everyone?

Mr. SOUMAN: Yeah.

KRULWICH: Really? Everyone?

Mr. SOUMAN: Yeah.

KRULWICH: I can't believe that.

Mr. SOUMAN: No, people - and that's - I mean, again, I really encourage people to try it out themselves. Walk blindfolded on a big field. The first 10, 20 steps you think you're pretty good at walking a straight line, and actually you are. But then if you walk a bit longer and make 40, 50, 100 steps...

KRULWICH: Inevitably, he says, people at some point turn, and then turn a little more...

Mr. SOUMAN: Then people start walking in circles.

KRULWICH: Circles?

Mr. SOUMAN: It's really fascinating because people initially don't believe that they are walking in circles, but everyone walks in circles.

KRULWICH: And they have no idea that they're going...

Mr. SOUMAN: And they have no idea. They were thinking that they were walking in a straight line all the time.

KRULWICH: This tendency has been studied now, for at least a century. On npr.org we animated field tests from the 1920s, so you can literally see what happens to men who are blindfolded and told to walk across a field in a straight line, or swim across a lake in a straight line - or one was put in a car blindfolded and told now drive in a straight line - and they couldn't. In the animation, you see them going into these strange loop-de-loops in either direction.

Apparently, there's a profound inability in humans to go straight unless there's something in the environment you can see and focus on.

Mr. SOUMAN: Like a mountain or building or the sun.

KRULWICH: If you can fix your eye on one of those...

Mr. SOUMAN: Then people were able to walk in a pretty straight line.

KRULWICH: But without external cues, there's apparently something in us that makes us turn, but what?

Well, one obvious explanation would be this is some kind of handedness, like right-handed people favor their right side, lefties their left side, so maybe this is just some version of that. But when Jan's team sorted the subjects into lefties and righties...

Mr. SOUMAN: We didn't find any correlation between in which direction they went or whether they were left- or right-handed.

KRULWICH: So righties went to the left or the right, and lefties went to the right or the left, it didn't matter.

Mr. SOUMAN: Exactly. Exactly.

KRULWICH: Well, what about this possibility? We all know our brains have two sides. We have left brains and right brains.

Mr. SOUMAN: Um-hum.

KRULWICH: So let's say that there's maybe more oomph or neurotransmitter on one side of our brains that pushes us in a particular direction.

Mr. SOUMAN: Right. Yeah, that's a very popular hypothesis.

KRULWICH: Except when they tested it, they couldn't find the pattern.

Mr. SOUMAN: So that kind of suggests that that's probably not really the cause of this phenomenon.

KRULWICH: Well then maybe - maybe it's very simple. Maybe in all of us one of our legs or arms is just a little bit stronger or a little bit longer than the other.

Mr. SOUMAN: Yeah.

KRULWICH: And that over time that little difference just builds up.

Mr. SOUMAN: Well, that's actually one thing we tried.

KRULWICH: Huh.

Mr. SOUMAN: So we actually put rubber soles underneath one foot...

KRULWICH: ...gluing them onto one shoe and then on the other shoe they glued a slightly thicker sole?

Mr. SOUMAN: Thicker than the other one. So we in effect created an unequal leg length.

KRULWICH: Oh good. So did they turn in the predictable direction?

Mr. SOUMAN: No, they didn't. It didn't make any difference at all. So again, that is pretty random what people do.

KRULWICH: Wow, so this is a pretty deep puzzle.

Mr. SOUMAN: Yeah.

KRULWICH: And whatever the explanation turns out to be - and we don't know that yet - it looks like there's going to be several causes, not just one. But the real lesson here is that something as simple as walking straight can turn into such a mystery, which is why doing science like this is so much fun.

Mr. SOUMAN: Right, yeah.

KRULWICH: Robert Krulwich, NPR News.

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