You can click your tongue or snap your fingers or anything really. It really doesn't matter what you use as a "clicker" as long as you reinforce that sound consistently in the same conditioning you would if you did have a clicker. Even so much as a "clever bird!" or other saying used EXCLUSIVELY for training is enough to act as a conditioned stimulus.The basis of clicker training is classical conditioning, and if you Google the origin of classical conditioning, their stimulus is a bell. The same can be said if you consistently click a pen and smack someone in the face. Sooner or later, they'll start flinching with the pen click even if you're not going to smack them - so that stimulus can be anything in the world you want it to be (haha, lame example I know).