Sheldon Teases Leonard Riddle

To tease Leonard Hofstadter, Sheldon cooper gave him 10$ and ask him to buy 
- Something their friend Howard can eat. 
- Something their neighbour penny can drink. 
- Something their friend Raj's cow can eat.
- Something his girlfriend Amy can plant in her garden. 
Also, he can buy only one thing. 

Leonard buys something that silenced Sheldon. What did he buy?

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