A timer and a simple rule: stop it at exactly four seconds.
Got this game idea while I found myself routinely trying to stop a timer at a 'round' value while at the gym. Also heard some stories about how people used to play this sort of stopwatch games on digital watches, back when Casios were the thing.
Everyone knows how to add two numbers, right? This is simple, the most basic mathematics taught to toddlers, and probably one of the most important things you may learn in your life. People even have professions based on this. This game is just that. With a beautiful looking interface and very simple concept, "Sum"...
The basis of Sum is simple. Everyone learns addition very early on in their schooling and every single day after that you begin adding something in your every day life. Since you have so many years of experience you should be amazing at addition, but are you?