If you live in a hard water area, limescale will build up in your kettle.
Fill the kettle, remove the filter (if there is one) and boil.
Measure out one dessert spoon of citric acid.
When the kettle has boiled, add the citric acid and put in the filter too.
Set a timer for 15 minutes.
After 15 minutes, check if the limescale has cleared - if some remains, repeat the process.
When all the limescale has gone, rinse the kettle a couple of times and put the filter back in.