When Rootbound Lake enters the battlefield, sacrifice it unless you pay .
: Add or to your mana pool.
Sacrifice Rootbound Lake: Add or to your mana pool.
: Rootbound Lake becomes a 2/2 green Elf creature until end of turn. It's still a land.
.
: Add or 
to your mana pool. or to your mana pool.
: Rootbound Lake becomes a 2/2 green Elf creature until end of turn. It's still a land.
