The brake fluid container of my 2010 Honda has started to bubble (clear reservoir).
It's boiling.. time to change your brake fluid!
Brake systems aren't completely sealed - and brake fluid is hygroscopic so over time moisture works it's way into the fluid (either through flexing on the brake lines or through the breather on the reservoir) and as the moisture level in the fluid increases the boiling point drops.
Boiling brake fluid is a Very Bad Thing - brake fluid is supposed to be incompressible (so it can transmit the braking force), the gaseous bubbles you get when it boils are way more compressible than the fluid so when you hit the brakes you compress the bubble, giving that horrible "spongy" feel - and that force doesn't get applied to the brakes, and if it's particularly bad you aren't going to be doing much slowing down. And because the effect is induced by having successfully applied the brakes and generated heat in the fluid you can go from "fine" to "no brakes" from one application of the brakes to the next - screaming, panicking and often crashing then ensue.