Dependency Injeciton - the cooking metaphor

Let's take a typical recipe. It first describes the goal - and then it goes:

Ingredients:

• 2-1/4 cups sifted cake flour
  
• 2 teaspoons baking powder
  
• 1/2 teaspoon salt
  
• 1/2 pound (2 sticks) sweet butter, room temperature
  ...
  

Preparation:
Preheat the oven to 350 degrees Fahrenheit. Butter and line two 8 x 3-inch baking pans or one 12 x 3-inch pan with parchment.

It is not:

Preheat the oven to 350 degrees Fahrenheit. Find a cow and milk her, wait until ...

neither it is:

Preheat the oven to 350 degrees Fahrenheit. Take your credit card and go to the grocery around the corner ...

Dependency injection is about writing your programs in a very similar manner - you first declare the collaborators and then go on with using them.