Gout

Salt crystallizing in a shrinking puddle

Imagine a puddle of salty water on a hot driveway. The salt (uric acid) stays dissolved as long as there's enough water and the temperature stays warm. But if you pour in more salt faster than it can drain away (overproduction), or if the drain gets clogged so water evaporates faster than it leaves (underexcretion), the puddle shrinks and gets saltier. Once the salt concentration crosses a threshold, crystals suddenly form and scrape your bare foot (the sharp needle crystals in your joint). Your body's immune system rushes in trying to sweep away the crystals, but instead stabs itself on the sharp edges and causes an angry, swollen, burning mess (the flare). If you never fix the drain or stop pouring salt, the puddle keeps shrinking, more crystals form, and eventually the whole driveway gets crusted and cracked (chronic tophi and joint destruction).