Danilka was able to say that there is no secret in nature that baffles Terenty because Terenty was able to manage all his questions about nature. Terenty was able to answer everything he asked about the secrets of nature.
Just like when Terenty compared the nightingale and the sparrow to a singing-bird without a sin and a spiteful bird (respectively).
Another one is the swarm of bees flying, looking for home. He informed Danilka that, to make them settle, all he need to do is to sprinkle them with water.