The strange liquid is a utility item that turns Bob invisible. While invisible, Bob will not alert anyone of his presence but the liquid does not make him completely silent, meaning that any noise caused by Bob will still alert the attention of nearby guards. It can be used to escape from guards. However, the liquid will not fool a Dog.
It costs 100 coins to buy one bottle of it.
"includes all the vitamins a growing thief needs! it'll only last a couple of seconds though."
-Strange liquids description.