Go to cycling road from Celadon City and go the the left border with the trees behind the stone pillars. Go down the hill for about 1 second and go back up. The trees, up to behind the guy two spaces away from the border, will have turned into water! Go back down(or to the right and back left) and there will be trees again! Note: this only works between two points: the biker specified above and a pillar to the right, the last one before going futher right to border the lake.