def cross_product(v1, v2):
    """ Return the cross product of v1, v2.

    v1, v2 - 3d vector expressed as a list of 3 floats.
    """
    x3 = v1[1] * v2[2] - v2[1] * v1[2]
    y3 = -(v1[0] * v2[2] - v2[0] * v1[2])
    z3 = v1[0] * v2[1] - v2[0] * v1[1]
    return [x3, y3, z3]