The mystery of how a type of lizard "walks" on water may have been solved, a group of US scientists believe.
The basilisk lizard runs across the water on two legs
The basilisk lizard - also known as a Jesus lizard - has a seemingly miraculous ability to scurry across liquid, apparently at odds with the usual laws of physics.
Apart from a few types of spider and insect - such as pond skaters that are light enough to avoid piercing the surface tension of the water - the lizard (Basiliscus plumifrons) is the only creature that can perform this mystifying trick.
Harvard University's Dr Tonia Hsieh told the BBC World Service that experiments showed the lizard to be producing massive sideways force to stay upright.
"We did expect that we would see that they were producing enough force to run on the top of the water," the researcher in the institution's department of organismic and evolutionary biology explained.
"What we didn't expect to see was very large medio-lateral forces; forces pretty much to the side of the lizards."
The study, which was reported recently in the Proceedings of the National Academy of Sciences, reveals how a large upward force is produced every time the lizard slaps its foot down into the water.
This keeps the animal from sinking straight down into the liquid. But just like we tend to teeter forward when we run on a soft surface such as sand, the lizard would also stumble forward unless it had a mechanism for stabilising itself.
And this is where the sideways force comes in - and it is almost as strong as the initial slap down.
The findings are interesting because most of what we know about how animals with legs move is based on studies of them travelling across solid surfaces.
Animals that run on land with two legs, such as birds and humans, have little force directed out towards the sides. The basilisk lizard is very different.
"We were wondering why this is actually happening," Dr Hsieh added.
"Our guess on this is that it appears to help maintain stability... as they're running across water; they're constantly tripping.
"It's a matter of catching themselves and keeping themselves upright before they actually fall over."
The 'squishy' extreme
The lizards have long bodies and large feet. On the edges of their toes are fringes that resemble pom-poms with a number of fronds.
The experimental set-up used a small track, around a metre in length, with small, silver-covered reflective particles dropped in the test-tank water.
A laser light was then shone through the water, making the particles reflect. This allowed the scientists to visualise fluid flow induced by foot movements and to calculate the forces the lizards were producing.
Dr Hsieh said there were "definitely" broader implications coming out of the research that would centre on the study of how animals moved over many different types of surface.
"They have to manoeuvre across all these surfaces on a daily basis - how exactly are they doing this?" she said.
"You would suspect that, if you look at how they move across these surfaces, there would be a continuum in the strategy that they're using - but we don't know exactly what this strategy is.
"So if we take a look, and know what is happening at one extreme - solid ground - what if we look at the other extreme, in terms of a squishy surface, such as water?
"Once we figure that out, maybe we can start filling things in in-between, as opposed to individually testing the thousands of different combinations that there are out there, to try to figure out how they move on every different surface."