When you create a variable Swift can figure out what type the variable is based on what kind of data you assign to it, and from then on that variable will always have that one specific type.
Think about it: we just created one variable then tried to change its type, which will obviously fail. But as your programs grow in size and complexity, it becomes impossible to keep the types of your variables in your head at all times, so we’re effectively shifting that work on to Swift instead.
