How do they survive?
Cacti and succulents have adapted to grow where there is not much fresh water, in places like deserts. Deserts get very little rain, are very hot during the daytime and often frosty at night. A prickly cactus stops birds and animals trying to eat them for their water!
About cacti
Most plants get rid of litres of water every day, by a process called transpiration, giving out water through their leaves (a bit like when we sweat!). Cactus leaves have become a lot smaller so they don't lose so much water; most have shrunk to spines or hairs but some have disappeared altogether!
The stems are green to make food for the plant, because the leaves can't do it any more - they are too small.
Cacti stems and ridges expand to store lots of water when it rains. Think of a camel that stores food in his hump! This guides as much rain as possible to the roots.