nihonzuki:~ $ irb
irb(main):001:0>
irb(main):002:0* # Ruby knows what you
irb(main):003:0* # mean, even if you
irb(main):004:0* # want to do math on
irb(main):005:0* # an entire Array
irb(main):006:0* cities  = %w[ London
irb(main):007:0]               Oslo
irb(main):008:0]               Paris
irb(main):009:0]               Amsterdam
irb(main):010:0]               Berlin ]
=> ["London", "Oslo", "Paris", "Amsterdam", "Berlin"]
irb(main):011:0> visited = %w[Berlin Oslo]
=> ["Berlin", "Oslo"]
irb(main):012:0>
irb(main):013:0* puts "I still need " +
irb(main):014:0*      "to visit the " +
irb(main):015:0*      "following cities:",
irb(main):016:0*      cities - visited
I still need to visit the following cities:
London
Paris
Amsterdam
=> nil
irb(main):017:0>