[header] How to clean hard water spots off windows [title] Fill a squirt bottle with half water and half vinegar. [step] The best vinegar to use is plain white distilled vinegar, which is inexpensive and effective. A higher concentration of vinegar may be even more effective, so adjust the vinegar to water ratio based on the severity of the water spots. 
[title] Spray the vinegar solution on the glass. [step] Spray the solution on the glass, being sure to fully saturate any stubborn water spots or areas of concentrated buildup.