[ruby] How to split (chunk) a Ruby array into parts of X elements?

I have an array

foo = %w(1 2 3 4 5 6 7 8 9 10)

How can I split or "chunk" this into smaller arrays?

class Array
  def chunk(size)
    # return array of arrays
  end
end

foo.chunk(3)
# => [[1,2,3],[4,5,6],[7,8,9],[10]]

If you're using rails you can also use in_groups_of:

foo.in_groups_of(3)