What do cactus drink?

They use photosynthesis to produce food, they are producers!

Above the ground a cactus absorbs the energy from the sun to convert carbon dioxide into sugars and other compounds. Below ground they have sophisticated root systems which enables them to collect water and nutrients, even from very poor soil conditions. When grown in pots, in special cactus soil, the occasional supplement of a weak fertiliser will help to regenerate the soil. Tomato fertilizer can be used, but at one-third of normal strength. However, the danger with cacti is overfeeding! They prefer arid conditions, and are more likely to flourish and flower when their environment is appropriately restrictive!