^ This.
Make sure that you are using sterile water. Bacteriostatic water is ideal, but you can use bottled water which may be more practical. You need to use a new, unopened bottle each time though, because once you open it some germs and bacteria will get into it, making it unsterilized for next time. If you are ever stuck using tap water (which you should never do) use cold water, boil it for 30 minutes, and then let it cool back down before using it. Tap water contains chlorine, fluorine, orthophosphate, and sodium hydroxide, which is why you should not use it to prepare IV solutions.