Ever walk into a restaurant or see food that's been left out at work and instantly start licking your lips? The mouthwatering sensation some people experience when they see or think about something delicious is actually a reflex—you can't control it. All it takes is a whiff or just the thought of food, and bam, you start to salivate. The only caveat is that you have to believe there's a chance you'll get to eat the food. So scrolling through your favorite food blogger's Instagram feed may make you hungry, but it won't be (literally) mouthwatering.