Can I re-boil water for tea?

It's real, but the effect is small. Boiling drives off dissolved oxygen, which gives water a fuller, livelier mouthfeel. Re-boiled water tastes 'flat' to trained tasters — especially in delicate greens and whites where the water itself is half the experience. For pu-erh or black tea you'll barely notice. The fix is easy: start with cold filtered water, boil once, use what you need. Don't re-use the kettle's leftover boil from this morning.