You place six jars (left to right: coffee, arsenic, and sugar on the top shelf; snuff, tea, and salt on the bottom shelf) and have somebody else fill them up with these ingredients, making sure each jar contains something other than what the label says.
You are told that
the salt is located under the jar containing snuff, and that the sugar
is located directly to the right of the jar containing coffee.
Where is the arsenic?