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Methods are defined with def ... end. Parameter types aren't declared. The last evaluated expression is implicitly returned (no return needed).

def square(n)
    n * n   # last expression returned
end

puts square(5)   # 25

Default values, splats, and keyword arguments make signatures very flexible:

def greet(name, greeting: "Hello", punct: "!")
    "#{greeting}, #{name}#{punct}"
end

greet("Alice")                       # 'Hello, Alice!'
greet("Bob", greeting: "Hey")        # 'Hey, Bob!'

def sum(*nums)   # splat — variable args as array
    nums.sum
end

sum(1, 2, 3)    # 6

Method names ending in ? typically return a boolean. Ending in ! typically mutate the receiver or raise on failure.