Why are leaves modified to form spines in the cactus plant how then does the cactus plant make food?

The cactus adapted to its harsh environment by developing thick skin to slow the evaporation of water. The leaves changed in order to reduce the surface area that water could evaporate from and over time became defensive spines to protect itself from animals trying to get inside its thick skin for the moisture.

The cactus makes its food, by having chlorophyll throughout the rest of plant. That is why the entire cactus is green.