The reason pouring hot water into the sink may not work as expected is because of how grease and plumbing systems interact. Here's why:

1. Grease hardens when cooled: Hot water may initially melt grease, but as it cools down, the grease can solidify again, especially if the water doesn't reach high enough temperatures or if the grease is in areas that are already cool (like pipes further down). This can cause the grease to build up and eventually block the pipes again.

2. Hot water can damage pipes: If the water is too hot, it can weaken or even damage the seals and joints in older plumbing systems, leading to leaks or further clogs. This is especially a concern with plastic pipes, which can soften or warp when exposed to extreme heat.

3. Temporary relief, not a permanent solution: While hot water might temporarily melt the grease, it doesn't fully solve the problem. Over time, the grease can build up again, leading to recurring clogs.

A better solution:

To effectively clean grease and prevent clogs, it's better to use a degreaser or a drain cleaning solution or try baking soda and vinegar. Regularly scraping grease off dishes before washing them also helps reduce buildup in the pipes.