Prevent non-numeric character inputs

> I know what this group needs.
How 'bout we grab some lunch and watch the "capture the flag" game?
You come watch the "capture the flag game" with the normal kids, and then you can have the whole rest of the day to learn about planets, stars, pulsars, heliocentricity, gravitational collapse... and the science of celestial mechanics... as shown through the work of the 19th century scientists... Alexis Clairaut, Jean d'Alembert and Pierre Laplace.
Okay? Ready?