There’s an art–some would say a science–to food dipping.
One must know the attributes of the item being dipped.
Oreo cookies, for example, must be either dipped with a fork inserted into the creamy filling, or “reverse dipped,” by filling a cup with Oreos and then pouring milk over them. 2% milk is preferable, as it complements the creaminess of the Oreo without freezing the mouth the way skim milk does.
Grilled cheese sandwiches should be dipped in soup. Some experts insist that only tomato soup is appropriate for cheese dipping; I am not so narrow-minded.
Now potato chips–well, what can’t they be dipped into?
On the West coast of the U.S., ranch dressing is a surprisingly popular dip. I can speak from experience when stating that it’s suitable with pizza, chips, fries, hamburgers, cheese, chicken strips, and other items too numerous to mention. (Experimentation is recommended.)
Note that for foods with layers, such as pizza, care must be taken to ensure the layers do not fall into the dip. A stray olive in otherwise unsmirched guacamole may be tolerated; the entire contents of a taco is unforgivable.
Of course, it goes without saying, but double-dipping is only acceptable when both parties favor the practice.