زجاجة ماء

Took this to the climbing gym and the 500ml size fits perfectly in my pack's side pocket. The membrane lid seals tight enough that I can toss it in my bag without worrying about leaks.

The membrane lid really is a one-hand open and close, which I didn't believe until I tried it while actually riding. It stays sealed in my bag too, no leaks so far.

That membrane lid is genuinely a one-hand operation, which I didn't expect to actually work as well as it does. I keep it clipped to my bike frame and never have to fiddle with a cap when I'm stopped at a light. The 500ml is just right for a short ride without weighing me down.