text: If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you don't reveal the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell; and if you play a land from the top of your library, you can't look at the next card until you're done handling any replacement effects that modify how that land enters the battlefield (such as that of Vesuva).