Why did the ancients put people in coffins after death? Why nail the coffin lid?

Coffins are the shelter of the deceased, and coffins are generally made of high-quality cypress, pine, willow, paulownia and other materials. In ancient times, there was no need to nail the coffin. The bottom of the coffin and the lid were tied together with a thong. Hit three horizontally and two vertically. This is because the horizontal board is long and the vertical board is short, which is also the reason why the idiom is called three long and two short later.