Well, I fell. Stupidly. And when I landed, on my left knee, I felt a deafening "squish"... sqshlktsht
I thought... that can't be good. But oddly, it didn't hurt - it just felt incredibly weird. And then a purple grapefruit grew out of my knee. Obviously I iced it, but here's what happened: I ruptured my subcutaneous infrapatellar bursa. The little fluid sac beneath my knee cap - all these sacs hang around in your joints to prevent friction between bones and ligaments and such, and also to protect the joint from external forces (like landing hard on one's knee) - sort of like an air bag - only with fluid instead of air.
So basically, when my knee grew a grapefruit, this was the air bag going off.
The cool thing is how these bursae heal themselves, and before too long all the fluid will go back into the healed sac and my knee will be good as new again! Hoorah! Meanwhile, there's still some swelling and some significant bruising (I think it's funny that's it's a doughnut shaped bruise), I didn't take a photo of the grapefruit, or even the navel orange that followed the next day after I taped an icebag to it for the night. Once the left knee bulge has become a dent again like the right knee - all will be well again! Aren't bursae neat?