Despite its name, the heliosphere's shape is not a perfect sphere. Its shape is determined by three factors: the interstellar medium (ISM), the solar wind, and the overall motion of the Sun and heliosphere as it passes through the ISM. Because the solar wind and the ISM are both fluids, the heliosphere's shape and size are also fluid. Changes in the solar wind, however, more strongly alter the fluctuating position of the boundaries on short timescales (hours to a few years). The solar wind's pressure varies far more rapidly than the outside pressure of the ISM at any given location. In particular, the effect of the 11-year solar cycle, which sees a distinct maximum and minimum of solar wind activity, is thought to be significant.