Surface tension is the force that holds the surface of a liquid together, creating a barrier against external forces. It arises from the unbalanced molecular forces at the liquid's surface. In the case of water, its high surface tension is attributed to the strong intermolecular forces known as hydrogen bonds.

Hydrogen bonding is a dipole-dipole interaction that occurs between highly electronegative atoms (such as oxygen or nitrogen) and hydrogen atoms bonded to these electronegative atoms. In water, the oxygen atoms in the molecule are partially negative due to their high electronegativity, while the hydrogen atoms are partially positive. This polarity allows water molecules to form strong hydrogen bonds with neighboring molecules, creating a cohesive network.

Water molecules at the surface experience an imbalance of intermolecular forces. On one side, they are bonded to other water molecules within the liquid, whereas the other side is exposed to air or another immiscible liquid. The strong hydrogen bonds between water molecules at the surface pull them closer together, minimizing the surface area and reducing the interaction of water with the surrounding environment. This cohesive behavior results in a higher surface tension for water.

In contrast, methane is a nonpolar molecule, meaning that its electrons are evenly distributed and there is no significant electronegativity difference within the molecule. As a result, methane molecules lack the ability to form strong hydrogen bonds with one another. The intermolecular forces in methane are primarily London dispersion forces, which are weak attractive forces arising from the temporary fluctuations in electron density.

由于甲烷分子之间的范德华力较弱，甲烷的表面张力小于水。

Therefore, the absence of strong intermolecular forces in methane leads to a weaker cohesive behavior at its surface. Methane molecules experience weaker van der Waals forces, which are insufficient to hold the surface together as tightly as in the case of water. This results in a lower surface tension for methane compared to water.

In summary, the high surface tension of water is a consequence of the strong hydrogen bonding between its molecules, which creates a cohesive network and minimizes the interaction of water with the surrounding environment. On the other hand, methane lacks these strong intermolecular forces, leading to a lower surface tension due to weaker van der Waals forces.