Tepid water is lukewarm, meaning that it is neither hot nor cold to the touch. The American National Standards Institute (ANSI) stipulates that tepid water should have a temperature from 60 to 100 degrees Fahrenheit or 16 to 38 degrees Celsius. Tepid water is preferable to hot or cold water in a variety of instances; for instance, it is required in emergency showers and eyewash stations because it will neither scald nor freeze a victim of chemical burns, and it is optimal for watering a variety of houseplants because the lukewarm temperature will not shock their roots with excessive heat or cold.
Leave the desired amount of water out overnight at room temperature. By morning, the water will have cooled or warmed up to a tepid state.
Heat water to a tepid temperature more quickly using a microwave. Heat small quantities of water at 50% power for 20-second increments until the water is either slightly warm or slightly cool to the touch. You may also add small quantities of hot water to the cold water until it is tepid. Do not attempt to heat the water over the stove, as you may burn yourself testing the temperature if you overheat it.
Add ice to hot water if you wish to make it tepid quickly. Drop the ice into the water and check periodically to see if the water has become tepid. Once it is tepid, fish the ice cubes out. You can also put the water into the refrigerator or freezer until it has cooled.