Human: What is the difference between a saguaro cactus and a prickly pear cactus? Assistant: 
The main difference between a saguaro cactus and a prickly pear cactus is that the saguaro cactus has a thick, woody stem, while the prickly pear cactus has a thin, spiky stem. Saguaros also have spines on their arms, while prickly pears do not. Additionally, saggars are often larger than prickly pears, with some reaching up to 30 feet in height.