why is the water diffrent than other things, when it comes to thermal sepansion?

我觉得是扩散现象
扩散是由于微粒（原子、分子等）的热运动而产生的质量迁移现象，主要是由于密度差引起的。在扩散过程中，气体分子从密度较大的区域移向密度较小的区域，经过一段时间的掺和，密度分布趋向均匀。在扩散过程中，迁移的分子不是单一方向的，只是密度大的区域向密度小的区城迁移的分子数，多于密度小的区域向密度大的区域迁移的分子数。

扩散现象是气体分子的内迁移现象。从微观上分析是大量气体分子做无规则热运动时，分子之间发生相互碰撞的结果。由于不同空间区域的分子密度分布不均匀，分子发生碰撞的情况也不同。这种碰撞迫使密度大的区域的分子向密度小的区域转移，最后达到均匀的密度分布。

Thermal expansion is the tendency of a matter to change in volume in response to a change in temperature. When a substance,in your case, water, is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature.

Common engineering solids usually have thermal expansion coefficients that do not vary significantly over the range of te