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?
From the clues we have that the snuff is right above the salt. The pair can't be at the far right of the cabinet, since that would be the correct position of the salt (none of the substances can be in their proper container) and they can't be in the middle because that would in turn separate the sugar and coffee, which are beside each other. So, the snuff is on the top shelf, far left. The salt is beneath that. The coffee is on the middle of the bottom shelf and the sugar is to the right of it. Hence, the arsenic must be in the container in which the sugar belongs.