JeffreyPSA: you might experience a case of shiny object syndrome. Your mind is trying to not work on the task at hand, which is probably more important and/or urgent.
If so, write down your idea and thoughts about it, and set it aside. When you've done the task(s) at hand and come back to it, you'll have more energy and motivation for your 'shiny object'. Or you'll find out it wasn't important enough to chase.
In both cases, you've done some actual work today, congrats!