Overflow sensors are just about the easiest device you can make. Essentially, the design is similar to a switch, wherein the flow of electricity through the circuit is broken. An example application is aquaponic grow beds being flooded: when the water level meets both probes, the circuit is complete, the flow of electricity resumes and is detected by a digital pin as an overflow occurring, which in turn can shut off the pump. A state change of the pin, from LOW to HIGH or HIGH to LOW prompts an update to Tracker and, if the new state is HIGH and you want alerts, sends an email alerting you to a potential overflow. The probes used in this tutorial are stainless steel machine screws to minimize corrosion, and they are inexpensive and easy to replace.