What happens to a creature if its toughness becomes 0 or less?

When a creature's toughness becomes 0 or less, it is immediately put into its owner's graveyard as a state-based action. This rule is clearly defined in the comprehensive rules of Magic: The Gathering. Specifically, if a creature has 0 or less toughness due to damage, effects, or other changes in its stats, it is no longer considered to exist on the battlefield. Such creatures do not remain in play, nor do they get exiled or returned to the hand. Instead, they are sent to the graveyard, which effectively removes them from play. This mechanism ensures that only creatures with positive toughness can continue to exist on the battlefield, maintaining the integrity of gameplay.