The term sterile water actually means two different things. Many people equate sterile water as drinking water, and you can make drinking water by merely boiling water in an open pot, using a filtration device to remove harmful bacteria or adding in some type of chemical. All of these methods will turn contaminated water into perfectly fine drinking water. But for scientific research that requires truly sterile water, there is only one accepted way.
Fill a pressure cooker with water.
Place the pressure cooker onto a heat source, like a hob, and heat up the water inside.
Observe the temperature of the water inside of the pressure cooker. The water must boil and remain heated at a temperature of 121 degrees C, which equates to nearly 121 degrees C.
Maintain this temperature for 20 minutes to make truly sterile water.
An autoclave will also work in the same way, heating water past its boiling point of 100 degrees C, up to the required 121 degrees C.
Always use extra caution around heat sources and boiling water.