She finally made it!

At a girls' summer camp they were having a swimming meet all around the lake. the breast stroke was chosen, all the girls competing got ready in the water and with "...three...two...one..GO!" they were off. About a half- hour later they all reached the finish within a short time of each other, except for Marie, who wasn't to be seen. They were getting worried when, after another half hour, Marie finally came toward them, moving very slowly and smoothy, with deep breaths of exhaustion. They all asked, "What took you so long?-- we made it back a half-hour ago." Marie said, "A half-hour ago?? I think you all must have cheated and used your arms and legs!"