A small, flat-bottom, glass-wall kettle. That way people who always fill the thing completely aren't being as grossly wasteful as they might be. The glass wall

A small, flat-bottom, glass-wall kettle. That way people who always fill the thing completely aren't being as grossly wasteful as they might be. The glass wall helps other types see exactly how much water they need. The flat bottom means you can boil one small cup of water if that's what you need.