def delta_r(step):
    """Cells thickness.

    Args:
        step (:class:`~stagpy._step.Step`): a step of a StagyyData instance.
    Returns:
        tuple of :class:`numpy.array`: the thickness of the cells and radius.
    """
    edges = step.rprofs.walls
    return (edges[1:] - edges[:-1]), step.rprofs.centers