Creatures you control with power or toughness 1 or less can’t be blocked.
"Her talent for slipping unseen behind enemy lines provided information that would prove crucial to Dominaria's defense."
—*Heroes of the Second Invasion*
Once a creature you control has been blocked, changing its power to 1 or less won't cause it to become unblocked. Changing its toughness to 1 won't cause it to become unblocked, and changing its toughness to less than 1 will cause it to die.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.