Current norms state that a watch can only be called "Water-resistant" if it withstands perspiration, drops of water, rain, or immersion in one metre of water for 30 minutes. Superior levels of water-resistance are generally indicated on the dial or the case in the form of bars of pressure or a depth in meters. Diving watches are subjected to even more stringent tests. They must be capable of being used for at least one hour a day at a depth of 100 meters. It is important to remember that the water-resistance of a watch is tested at a particular point in time and is not guaranteed for life. Any work on the case, or strong changes of temperature (e.g. taking a cold shower after lying in the sun), as well as the ageing of materials can lead to moisture penetrating the most water-resistant watch.