Soil is a three phase system.

Soil is known as three phase system but why???
when we treat with soil some time we feel its behavior like rigid material or sometime it's like loose like sand some tims its behavior like leqid it can be flown like water.
if we deep drive to soil for understanding so basically it has three parts .
soil solid that is rigid.
water part
air part.

combine of watet and air known as void.
we usually consider volume of air, water ,soil solid
and mass of water and soil solid.

that's why soil known as three phase system.

link of video 

https://youtu.be/35rGCZCEV8c