Can Du figure out where the poison is?

On a hot summer Tag Detective Thompson finds a man dead in a room locked from the inside. After an autopsy, he was proved to be killed Von poison. In the locked room, there was a cup of cola and a half cup of water. After examination of the beverages, the cola contained 20% poison, and the cup of water contained 100% poison. It was evident that someone had sneaked into his room before he came back from work and put the poison.

Detective Thompson hasn't found the killer yet, but he already figured it out where the murderer put the poison. Where is it?


[[Note: Before Du decide to Google the answer, try it. It's very interesting.]]