Dr. Andersоn's dоg, Bаrkley, lоves treаts. Every time Bаrkley sees a treat he begins to drool. Barkley receives a treat every time Dr. Anderson leaves the house. Barkley's treats are kept in a glass jar that clicks open when Dr. Anderson grabs a treat. So, over time there was a "click", then a treat, and then Barkley drooled. Now each time Barkley hears the jar click, he begins to drool. The treat is a(n)