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People who want to make tea or coffee on Shabbat can plug in an electric urn to heat up the water before Shabbat. When they want a hot drink, they must pour hot water from the urn into a cup. Then, from this cup, they must pour the water into another cup. Only then may they place the tea bag (or coffee crystals) in the water.
Why is this permissible? Normally, food and water are cooked, using a fire or electric coil, in a pot or pan (kli rishon). From there, the food or water may be poured into a cup (kli sheni) since we generally assume a kli sheni is not hot enough to cook anything. However, there is a concern that certain easily cooked foods might be cooked even in a kli sheni. This is why the practice is to pour from there into another cup (kli shlishi), which certainly is not hot enough to cook anything.
