Idea: An audio alert that notifies you when the shower water has heated up and is ready for you to start showering (which can take over a minute in some cases). Also: it could maybe attach to the shower head and whistle like a teapot when the shower water is ready; you could also put a sensor in the water heater to detect when you’ve run out of hot water and then to give you a “two minute warning” as you use the last of the hot water which is still in the pipes; attaching a dental water pik to a shower head, and (jokingly) using it as a bidet
