Demonstrates the difference between Ruby's two different block styles.

blocks.rb

def a(*args, &block)
  puts "a got a block" if block_given?
end

def b(*args, &block)
  puts "b got a block" if block_given?
end

# In this call, `b' is called with the block and the
# return value is given to `a' as an argument.
a b {}

# In this call, `a' gets the result of `b' *and* the block.
a b do
end

To clarify, mjijackson is showing that teh Ruby parser binds {} more tightly than parameters, which bind more tightly than do...end. Excellent example!