No matter how hard they try, astronauts can't whistle while they work. Why? It has to do with the air pressure in their spacesuits.
From ABC News:
So why can't people whistle while spacewalking? [Former astronaut Dan] Barry said there is a simple explanation.
"You can't whistle because the air pressure in the suit is only 4.3 [pounds per square inch], and normal atmospheric pressure is 14.7 psi, so there are not enough air molecules blowing by your lips to make a sound," he said.
What would Barry have whistled if he could have? "'Whistle While You Work,'" he said.
As io9 points out, astronauts will have to stick to humming.
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Image: Buzz Aldrin on the surface of the moon on July 20, 1969 via NASA