Their transparent body makes daphnia uniquely useful for testing how substances affect a living body. You can watch their heart beat directly under a microscope, with no dissection required. When you dissolve common over-the-counter medications — pseudoephedrine, aspirin, and Benadryl — into separate water solutions and record heart rates, each drug's effect on a real heartbeat becomes measurable. The control group in plain water gives you a baseline, and slow-motion video playback from a camera attached to the microscope makes counting heartbeats precise.

Using see-through water fleas to test substances lets you measure the effect on a living body with a simple count. Daphnia placed in increasing amounts of Red Bull show faster heart rates at higher concentrations. Those in full-strength Red Bull averaged 268 beats per minute, compared to 190 in pure water.