Two possible explanations based on elementary facts of physics are suggested for the universal preference for place of articulation of implosives and ejectives. Languages show a preference for ejectives in the order: velar, alveolar, and labial while implosives occur most often in the opposite order. A language will only have velar implosives if it has alveolar and labial. It will only have labial ejectives if it has alveolar and velar. The fact that this tendency holds in so many languages suggests a phonetic basis. Various explanations of this phonetic basis are summarized and two new hypotheses are suggested. The first of these is based on the action of the larynx in changing air pressure. The other involves the different sizes of the cavities created by closures at the different places of articulation and the differences in ability to change the size of those cavities. (PMJ)