Simple themes often result in excellent games - here's a good example, though owners of Stix will recognise the theme.
Everything starts easily enough with one escaped lepton bouncing round the walls of your research lab. You must trap it by walling up portions of the lab and eventually the lepton itself. If the lepton hits the ion-trail wall you're working on before it's completed, then bang goes a life. There are two speeds of build; slowly annexed portions are worth more than fast-filled areas.
As well as points for the areas captured and leptons caught, there is a time bonus for speedy completion of each screen. On the second screen, a chaser appears and tracks you round the walls - much slower, but equally as lethal as the leptons. With further screens, more leptons and more chasers appear until there are five of each to cope with.
Although it is a game for quick thinking, it is not solely dependent on chance. Leptons always bounce at 45 degrees off walls, so strategy and planning are possible, and pay dividends. That for me is what makes it so playable.