Game of Riddles

Look at the top 5 cards of your library and
separate them into two piles. An opponent
chooses one of those piles. If the chosen pile
contains a land, your opponent may draw a card.
You may cast cards from the other pile until end
of turn without paying their mana cost. Exile
both piles at the beginning of your next end step.
What has roots as nobody sees,
Is taller than trees,
Up, up it goes,
And yet never grows?