Once that happens, usually two or three hours later, depending on how warm your kitchen is, the whole mixture has become more starter.
How do you feed a sourdough starter? Simple. You add roughly the same amount of flour and lukewarm water as you have starter, mix it around so there aren't any clumps of dry flour, and let it chill out until things start bubbling up. Once that happens, usually two or three hours later, depending on how warm your kitchen is, the whole mixture has become more starter. (The warmer your kitchen, the faster it'll ferment.)