Science

Hi Monika,
Like all living things, cacti need water to survive. Given the areas where they live, though, water is often scarce. To compensate, cacti have developed special abilities that allow them to conserve the water they do receive and make it last a long time.
For example, the prickly spines of cacti are actually highly-modified leaves. Spines protect cacti from animals that eat plants and also help to reduce water loss by restricting air flow near the cactus.
Or understand that,
If you observe closely and compare cactus with other plants you'll find that cactus lack leaves. Instead they have spines. This is a special adaptation of cactus as well as other desert plants. These spine like structures helps preventing the loss of water and mineral content in the desert plants. The stem is typically succulent, meaning it is adapted to store water.
Photosynthesis takes place in the stems (which may be flattened and leaf-like in some species). Botanically, "spines" are distinguished from "thorns": spines are modified leaves, and thorns are modified branches. Cacti produce spines.
All the best.