Video: "I don't want to die." That's what Destin from Smarter Every Day says as his oxygen level drops dangerously low and yet he's completely unable to put his oxygen mask back on to save himself because his brain isn't functioning properly. Destin went to NASA to test what it's like to be inside a depressurised cabin in an aeroplane (that's when the oxygen masks drop down) to find out why you have to put your mask on first. The video of his experience is both fascinating and frightening. As the oxygen saturation levels in his body drops, he begins to experience hypoxia, and you can see how basic human functions (like say, the will to live) just completely stop working. In a matter of minutes, a normal, rational person devolves into someone who can't recognise shapes, someone who can't move his hands, and someone who can't save himself.
When your brain goes without oxygen, things can get ugly quick. Flying at 10,600m, you only have seconds of useful consciousness, so that's why it's so important for you to put your oxygen mask on first before you start helping others. If you tried to help someone else first, you could end up passing out before doing any good.