Over time mineral deposits build up on the inside of a tea kettle which make it less efficient and visually unappealing. 
Demineralize the situation with white vinegar.
Some prefer to dilute the vinegar with water before boiling it in the kettle. Take into account the degree of build-up and weaken or strengthen the mix accordingly.
Once the vinegar reaches a boil, turn the heat down slightly to maintain a controlled bubbling. 
After at least ten minutes, remove the kettle from heat and let the vinegar slightly cool. Rather than pitching the vinegar, consider using the warm solution to clean faucets or shower heads caked with limescale. 
This easy and inexpensive method leaves kettles sparkling. Afterward, be sure to boil a few rounds of plain water to rid the kettle of any lingering vinegar taste or smell. Coffee drinker? Vinegar works equally well to descale coffee pots. 
A good teapot is hard to find. After an extensive search, I landed on this glass Finum Tea Control.
In addition to having an elegant pour, this well-designed teapot allows you to twist close the tea from the water when you are done steeping to control the strength of your brew. It works with loose or bagged tea.
Even if tea isn’t your thing, this functional and elegant teapot makes a lovely gift. It is the perfect size for an afternoon tea party for two.