なんとなくirb

C:\work> irb
irb(main):001:0> 0.class
=> Fixnum
irb(main):002:0> nil.class
=> NilClass
irb(main):003:0> true.class
=> TrueClass
irb(main):004:0> false.class
=> FalseClass
irb(main):005:0> [].class
=> Array
irb(main):006:0> {}.class
=> Hash
irb(main):007:0> proc{}.class
=> Proc
irb(main):008:0> (1..3).class
=> Range
irb(main):009:0> 0.0.class
=> Float

なるほど、という感じですね。