How did she know?

So, the other night I had my groceries delivered. When I get deliveries, I put Baby Bug in the bedroom, so she doesn't sneak out. She's learned that she needs to go in there when I answer the door, and she goes in there easily. So, the other night, my groceries come. Baby Bug went into the bedroom. One of the things I ordered this time was wheat grass for her. I don't always order her grass, sometimes I do stop at the store and pick it up. So, I let her out of the bedroom, and what does she do? She runs immediately to her grass plate where the old grass still is. She didn't start eating the grass, she just patiently looked at me, until I got her fresh grass down for her. Then she dug in! She does love her grass! But how did she know that she had fresh grass coming? She's just too smart for her own good!