Having tea or coffee?Kettle manufacturers tell you never to under-fill their kettles - having them on when there is little or no water can damage the heating element so it is this small amount of extra water that we're talking about here.
There's two things you can do with what's water left over...
- Tip it into a jug and later use it for watering plants
- Sterilize your wiping-up cloth
BUT don't add extra water to your kettle just to allow the above, otherwise you're energy use becomes less efficient. Without adding extra water to the kettle, you'll still have at least half a mug-full left over.