Water boils when it begins to turn into steam. The bubbles are caused by the steam rising from the bottom of the saucepan to the surface.

The temperature of 212°F (100°C), which is normally given as the boiling point of water, is only correct if you are cooking at sea level. As you go higher the atmospheric pressure falls, causing the boiling point of water also to fall. Extra cooking time in both a saucepan and a pressure cooker is needed.

And that answers the old question: why can’t you make a good cup of tea on the top of Mount Everest?

The summit of Everest is nearly 30,000ft (9000m) high, and the atmospheric pressure there is less than one-third of the sea-level pressure. Water boils at only 158°F (70°C).

This temperature is not nearly high enough to extract the best possible flavour from tea leaves, and the result is a poor cup of tea.