Invalid w.r.t. prayer, fasting, and ceremonial washings. (An ablution or prayer may be invalidated by coming in contact with something ritually unclean, by touching s.o. of the opposite sex, by making a mistake in performing the ablution or prayer, by passing urine, feces, or gas, etc.).
When you are fasting you are not allowed to drink water because if you drink, your fasting will be invalid.