You must know the audience and try to figure out what they already know. You can refresh their memories briefly, but get to the point as quickly as your audience's knowledge will allow. Second, you can start too late. You have started too late if your first sentence has anything to do with an experimental manipulation. An introduction sets the stage and explains why you did the experiment you did.
Alcohol is one such substance.
Even if your experiment is completely novel and revolutionary, you must tie it into something your audience already knows about or cares about. The introduction of a talk locates your work in the body of other work. Before you can tell what you did, you must tell why. You must make the audience realize that your experiment is potentially an important one. You must intrigue them by showing the hole in the body of knowledge that your experiment serves to fill.