  We study an upper bound of ranks of $n$-tensors with size
$2\times\cdots\times2$ over the complex and real number field. We characterize
a $2\times 2\times 2$ tensor with rank 3 by using the Cayley's hyperdeterminant
and some function. Then we see another proof of Brylinski's result that the
maximal rank of $2\times2\times2\times2$ complex tensors is 4. We state
supporting evidence of the claim that 5 is a typical rank of
$2\times2\times2\times2$ real tensors. Recall that Kong and Jiang show that the
maximal rank of $2\times2\times2\times2$ real tensors is less than or equal to
5. The maximal rank of $2\times2\times2\times2$ complex (resp. real) tensors
gives an upper bound of the maximal rank of $2\times\cdots\times 2$ complex
(resp. real) tensors.
