If you've ever wondered why Mexican restaurants don't charge for chips and salsa, the reasons have as much to do with psychology as gastronomy.

Chips and dip is the perfect opportunity for a restaurant's waitstaff to upsell you on other items, especially drinks. The salty chips and picante salsa make you feel thirsty.

Rather than filling you up, chips and salsa can whet your appetite. A server uses this to strategically time their spiel of appetizers or specials and convince you to order more.

Eating something while waiting for your meal also keeps you from getting impatient. It's good hospitality, and your server hopes this translates into a generous gratuity.