Serene Squirrelling

At the beginning of each player's upkeep, you may untap any number of lands you control. Whenever a land untaps this way, put a growth counter on Serene Squirrelling. Lands with growth counters on them are 1/1 green Squirrel creatures with haste and "Sacrifice this creature: Add one mana of any color to your mana pool." They're still lands.