Your opponents can't cast spells with even converted mana costs. (Zero is even.)
Your opponents can't block with creatures with even converted mana costs.
8/25/2015 Effects that increase or reduce the cost to cast a spell don’t affect that spell’s converted mana cost.
8/25/2015 For spells with in their mana costs, use the value chosen for X to determine if the spell’s converted mana cost is even or not. For example, your opponent could cast Endless One (with mana cost ) with X equal to 5, but not with X equal to 6.
8/25/2015 For creatures on the battlefield with in their mana costs, the value for X is 0. Mistcutter Hydra (a creature with mana cost ) could block, but Endless One could not.
8/25/2015 The converted mana cost of a face-down creature spell or a face-down creature is 0. Void Winnower will stop face-down creature spells from being cast and face-down creatures from blocking.
8/25/2015 Yes, your opponent can’t even. We know.