Wait a second, this didn't make any sense. Dolphins weren't a type of fish at all, they were mammals. The "transform" part hadn't specified that, though, just the dare. Maybe she'd just touched the dolphin by random chance? In any event, something weird was happening to Jennifer's body. She watched in horror as the skin on her legs began to change color, turning from its normal healthy pink to a silvery gray. She gasped, startled, then panicked as her mouthed filled with water. She shot to the surface, coughing out the little water that had made it into her lungs, as her legs began to change.
Thinking quickly, Jennifer yanked the bottom of her swimsuit off, right before her legs began to merge together. From her waist downwards, skin slowly grew over the space between her legs, pulling them together as it constricted around her lower body. When that had finished, she could feel a second change make its way down her legs. The sensation of her skeleton rearranging itself wasn't actually painful, but it definitely wasn't pleasant. She could feel the bones of her now-hidden legs detach from her feet and slowly dissolve upwards, as her spine grew down the length of her forming tail, attaching itself to the abandoned feet, which had by now reshaped into a proper tail-fluke. As the last bits of her legs disappeared, leaving only her now-vestigial hips, Jennifer felt a strange sensation in her back. Craning her neck to see over her shoulder, she could just see the tip of the dorsal fin that had formed midway up her back, covered in the same gray skin as her tail.
And then it was done. The sensations of changing were over with, and Jennifer looked herself over. Nothing had changed, aside from the obvious; her lower body had become a dolphin's tail, but that was it. This blew; not only was she unable to walk on land now, she didn't have any gills or anything to supply the traditional mermaid's ability to breathe underwater. Which meant that she'd pretty much have to stay in the shallower parts of the ocean, which meant it would be harder to avoid unwanted attention.
A small fish passed by her, and Jennifer shot out a hand and brushed it as it went by. So the dare had taken effect after all! She waited for something to happen, but no other changes were forthcoming. What was up with that? Kyle had taken a breath in the middle of the "transform" part; maybe the bits aside from the basic transformation hadn't been counted? That was a mixed blessing, at best. Jennifer wondered what she should do from here; she wanted to go back and ask for help, but she wasn't sure that bringing attention to her new form would be such a good idea; what if someone besides her friends saw her, or what if one of her friends decided to try and capitalize on finding a real live mermaid? But she couldn't just live in the ocean; what was she to do?