Shape-selective catalysis, to put it simply, is the fusion of catalysis with the molecular sieve effect. Here, the shape or size of the reactant or substrate causes the catalyst to display preference or selectivity towards it. The size or form of the substrates and products, as well as the catalyst's pore structure, all affect these catalytic processes. Zeolites are an excellent illustration of this kind of catalyst. By definition, a zeolite is a "boiling stone." This is due to the fact that they are stones with very high heat retention rates. Zeolites have unique properties when heated because of their extreme porosity and ability to hold water in their pores. They have a high degree of stability and are resistant to extreme weather conditions, including high pressure and temperature.