In microgravity, says astronaut Chris Hadfield, "your eyes make tears but they stick as a liquid ball."

In May 2011, astronaut Andrew Feustel, in the midst of a seven-hour spacewalk, got something in his eye.

Feustel and fellow astronaut Mike Fincke had just finished running power cables from the U.S. side of the International Space Station to the Russian, and -- at the EVA's five-hour mark -- some anti-fogging solution from the inside of Feustel's helmet had begun to flake. One of those flakes, swirling in the modified snow globe that is a spacesuit helmet, ended up in precisely the place you wouldn't want it to.

"Just as an FYI, my right eye is stinging like crazy right now," the astronaut announced to his team. "It's watering a lot. Must have gotten something in it."

Fincke's reply? "Sorry, buddy."

Which was an appropriately mellow response, because Feustel's liquid little plight wasn't an emergency: He managed to wiggle down far enough in his spacesuit to reach the spongy device typically used to block the nose in case of a pressure readjustment -- and to use that to rub his eye. Which did the trick. But the episode was a reminder that much of the science -- much of the experimentation in general -- being done on the orbiting lab of the space station is being conducted, by default, on the humans who call the vehicle home. Knowledge -- about zero-gravity's effect on the body, about space as a human environment -- reveals itself in fits and starts, through unexpected events like the flaking of anti-fogging solution in an astronaut's helmet. In this case, Feustel's tearjerker was a reminder of the fact that astronauts, technically, can't cry.