You’re on a Pan Am space shuttle, headed for the spinning split-level O’Neil-colony space station in low Earth orbit. It’s a pleasant flight, and you’re relaxing to the strains of Johann Strauss’s Blue Danube waltz.
Suddenly, a monolith measuring exactly 1-by-4-by-9 crashes through the rear of your spacecraft, cutting the cabin in half. You’re strapped in to your seat, so the outrushing air doesn’t blow you out, and you open your mouth and let the air escape from your lungs so they don’t rupture.
But now, you’re in vacuum. You’ll asphyxiate in minutes if you don’t find something to breathe.
But lo! Dangling from the ceiling/wall/floor of the remains of your cabin is an emergency plastic hose, attached to a still-functioning oxygen tank! You strap it to your face and inhale. It’s pure oxygen gas.
Will you be able to get enough oxygen in your lungs to stay conscious, with no outside air pressing back on your lungs? And how long before other factors of being in vacuo get the better of you?