How does limestone absorb water?

Some forms of stone are igneous, essentially meaning that they were formed volcanically. Others are sedimentary, meaning that they were formed when various minerals/particles settle and compress into rock.

Limestone in particular is sedimentary, and is composed of skeletal fragments of marine organisms from the ancient seas--this makes the rock especially porous, and exposed limestone has a tendency to "honeycomb" in the presence of underground water or erosion.

The Texas Hill Country (where I grew up) is full of such stone, and local dams built in the first half of the 20th century often had to be re-planned several times over due to the porous rock in the original chosen construction sites. This is also why most cave systems are limestone.