What do you get when you cross a mosquito with a mountain climber?
Nothing. You can't cross a vector and a scalar.
EXPLANATION: A vector is a mathematical entity with both magnitude and direction in any number of dimensions. You can take the cross product of two vectors to form a new vector, similar to multiplication of real numbers.
A scalar is just a real number, a directionless magnitude in vector space. You cannot take a cross product of a scalar and a vector.
Hence, you can't cross a mosquito (disease vector) and a mountain climber (a scalar).
That is one terrible pun. I'm sorry.