What letter appears on a hot water tap in Spain?

In Spain, the hot water tap is usually marked with a red dot or a red letter C.

Here's why:

* Red: Red is internationally recognized as the color for "hot" or "danger".

* C: In Spanish, "caliente" means "hot".

However, it's always a good idea to double-check the tap before using it, as some older or less standardized taps may not follow this convention.