  One of the most important quantum algorithms ever discovered is Grover's
algorithm for searching an unordered set. We give a new lower bound in the
query model which proves that Grover's algorithm is exactly optimal. Similar to
existing methods for proving lower bounds, we bound the amount of information
we can gain from a single oracle query, but we bound this information in terms
of angles. This allows our proof to be simple, self-contained, based on only
elementary mathematics, capturing our intuition, while obtaining at the same an
exact bound.
