“Yawns are not consciously controlled,” says Robert Provine, from University of Maryland, Baltimore County. Even locked-in patients who cannot voluntarily move their limbs can yawn. Conversely, you can’t stifle a yawn. Once it starts, Provine writes, it “progresses with the inevitability of a sneeze.”

In humans, yawning is famously contagious; if you see someone else yawn, chances are you’ll do it too. A covered mouth will still spread a yawn, as will an upside-down face. Contagious yawning isn’t just limited to humans; it’s also found in other species like apes, monkeys, and dogs—domestic dogs can even catch yawns from their human owners.

For this reason, some scientists have suggested that yawning is a way of involuntarily showing empathy. After all, the species that have contagious yawns are all highly social. But the empathy explanation has been disputed, and regardless, it can’t be the whole explanation since yawning is also found in many solitary birds and mammals. It’s even possible (although more contentious) that fish yawn too. So it’s likely that contagious yawning is a more advanced form of a more evolutionarily ancient behavior that has some deeper physiological significance. What might that be? “There’s no consensus,” says Gallup. “It remains hotly debated.”