Why did the leaves of cacti change and take on the appearance of a needle?

The leaves of the cacti have changed into thorns to reduce the evaporation area, since cacti live in arid places. The second role of thorns is to protect against being eaten by animals.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.