Expensive perfumes come in tiny bottles, but many hide a whale-sized secret. To perfect a particular smell, perfume makers often use an ingredient that comes from whales with upset tummies. The whale waste, called ambergris, is solid and greasy on its own. But in perfume, ambergris holds all the smelly ingredients together for a long time. People describe its smell as “earthy” and “sweet”. According to the passage, the smell of many perfumes comes from _____.