Unexpected true/false behavior using defined?

truthiness.rb

THING = "thing_one"

puts THING

if defined? THING 
  puts "defined? THING is truthy"
else
  puts "defined? THING is falsey"
end

if THING == "thing_two"
  puts "THING == \"thing_two\" is truthy"
else
  puts "THING == \"thing_two\" is falsey"
end

# why is this truthy?
if defined? THING && THING == "thing_two"
  puts "defined? THING && THING == \"thing_two\" is truthy"
else
  puts "defined? THING && THING == \"thing_two\" is falsey"
end