Swift is a type-safe language. It checks to see whether you're assigning values of the correct type to variables and flags mismatched types as errors. Let's see how this works:

1. Enter the following code and run it:

// Type safety
restaurantRating = "Good"

Here, you are trying to assign a string to a variable of type Double. This is a type mismatch and you'll see a red circle with an exclamation mark inside it:

The exclamation mark means Xcode can't suggest a fix for this.

2. Comment out the line by typing two / characters in front of it so it no longer causes an error in your program:

// Type safety 
// restaurantRating = "Good"

Now that you know how to store data in constants and variables, let's look at how to perform operations on them.