When asked by someone "what they do", I could only answer that "you're seeing it."

Lithops (also known as "Living Stones") have two thick fleshy, rounded "leaves" that will crack open as they grow. This is for them to be able to increase in size. The new leaves (looking exactly the same as the original leaves) are underneath. Eventually, the original set of leaves will dry out (as the new leaves emerge) and then fall away.
You can see that in this photo.

Originating in South Africa, Lithops are one of the most difficult succulents to grow Sometimes they just die. No reason, it just happens. These plants are so strange and so unique that they are a fascinating challenge to have as part of a succulent collection.

The growth cycle of the Lithops is quite unique. There are basically 3 stages. The plant has no dormancy period (that is noticeable). In the Fall (sometimes Winter) a Lithop may produce a flower up between the leaves. However, I've only seen that happen once years ago with a plant that didn't survive past blooming. If a lithop flowers, all water must be discontinued until the plant has had a long rest. It is unclear "how long" the rest should be. Watering too much OR at the wrong time can be very deadly to lithops.

Temperatures below 50 degrees can also mean death to a lithop. I started this Winter with 5 containers of these plants. Because of the cold, I now only have two remaining. These are out in my Solar Room. Hopefully, these remaining two will continue to thrive.