פירוש על עבודה זרה 60:5

R. Safra classifies the types of snake poison based on the age of the snake.

R. Safra’s classification of snake venom seems to contradict a baraita that teaches that snakes grow stronger as they age. The Talmud resolves this by saying that while they go stronger, their venom grows weaker. That is why it floats to the top.