The concept of water expanding at 4 degree Celsius is not accurate. While it's true that water exhibits some unusual behavior related to temperature, its density reaches a maximum value at around 4 degrees Celsius but it does not expand.

Water molecules form hydrogen bonds with each other, creating a loosely packed structure. As temperature increases, the kinetic energy of water molecules increases, causing them to move faster and break some hydrogen bonds. This reduces the density of water, causing it to expand. However, this trend continues until water reaches its maximum density point around 4 degrees Celsius.

Beyond 4 degrees Celsius, the continued increase in temperature causes water molecules to move even faster, breaking more hydrogen bonds and allowing the water to expand again. This expansion is more pronounced at higher temperatures.

Therefore, water doesn't expand specifically at 4 degrees Celsius; instead, it reaches its maximum density at that point and begins to expand as temperatures increase further.