I have 2 little ones who characteristically have looser stools than the others. I classify diarrhea as runny/liquidy and wouldn't be concerned until the stools reach that point. I think it's totally possible to have diarrhea without showing other symptoms though...and since none of us are doctors I send home children if they have had 2 or more bouts of true diarrhea during a daycare day regardless of any other symptoms.