type of variable

var lightSwitchOn: Bool = true
var dimmer: Int = 7
var dimmerWithDecimals: Float = 3.14
var veryPreciseDimmer: Double = 3.14159265359

print(type(of: lightSwitchOn))
print(type(of: veryPreciseDimmer))

Bool
Double

Double, Bool, String, Float

use let to declare a constant when we expect its value to remain the same
we expect let not change

let encouragement = "You can do it!"

var personalizedEncouragement = "You can do it, Stephanie!"
personalizedEncouragement = personalizedEncouragement.replacingOccurrences(of: "Stephanie", with: "Ryder")

let birthYear = 2008
var currentYear = 2016
var age = currentYear - birthYear
age = currentYear - birthYear