Assigning return value to variable in Ruby in Learn Ruby the Hard Way, exercise 21

In Zed Shaw's Learn Ruby the Hard Way, exercise 21:

def add(a, b)
  puts "ADDING #{a} + #{b}"
  a + b
end

age = add(30, 5)  
puts "Age: #{age}"

This prints Age: 35.

I tried doing this with the previous exercise (ex20):

def print_all(f)
    puts f.read()
end

current_file = File.open(input_file)

sausage = print_all(current_file)

puts "Sausage: #{sausage}"

But when I run it, #{sausage} does not print, even after I move the file pointer back to 0:

def print_all(f)
    puts f.read()
end

def rewind(f)
  f.seek(0, IO::SEEK_SET)
end

current_file = File.open(input_file)

sausage = print_all(current_file)

rewind(current_file)

puts "Sausage: #{sausage}"

I assigned the return value from method add(a, b) to age, why can't I do the same with print_all(current_file)?

def print_all(f)
    puts f.read()
end

The return value of print_all is the return value of puts f.read(), which is the return value of puts, not the return value of f.read(). puts always returns nil. Therefore, print_all always returns nil.

Perhaps you intended:

def print_all(f)
    f.read()
end

Or if you need to print it in your function/method:

def print_all(f)
    foo = f.read()
    puts foo
    foo
end