while

while rand < 0.9
  puts "All work and no play makes Jack a dull boy."
end

begin
  puts "All work and no play makes Jack a dull boy."
end while rand < 0.9

until

until is the opposite of while

until rand > 0.9
  puts "All work and no play makes Jack a dull boy."
end

begin
  puts "All work and no play makes Jack a dull boy."
end until rand > 0.9

while and until both have one-line versions

puts "For he's a jolly good fellow!" while rand < 0.9
puts "For he's a jolly good fellow!" until rand > 0.9

for in

fruits = ["apple", "banana", "cherry"]
for fruit in fruits
  puts "I love #{fruit}!"
end

...but don't use for in! Instead use each:

fruits = ["apple", "banana", "cherry"]
fruits.each do |fruit|
  puts "I love #{fruit}!"
end