bottles_of_beer.rb

99.downto(1) do |i|
  puts "#{i} #{i.between?(2,99) ? "bottles" : "bottle"} of beer on the wall"
  puts "#{i} #{i.between?(2,99) ? "bottles" : "bottle"} bottles of beer!"
  puts "You take one down and pass it around,"
  puts "#{i} #{i.between?(2,99) ? "bottles" : "bottle"} bottles of beer on the wall!"
  puts 'No more bottles of beer on the wall :-(' if i == 1
  puts
end

Where i.between?(2,99) ? "bottles" : "bottle" evaluates to:

if i.between?(2,99)
  "bottles"
else
  "bottle"
end

This is known as a "ternary".