Awater molecule is made up of one oxygen atom and two hydrogen atoms why is it considered a pure substance

A pure substance is any substance on its own. For example, sand is impure for containing lots of different substances. However, diamonds are mostly pure for only containing one type of substance.

Whether a substance is pure or not doesn't relate to a substance's molecular structure, but different molecules in a substance. Water by itself is only H2O, therefore it still pure.

Water is not an element but a compound for containing different elements, however it still pure.

as the gila monster consumes the eggs or young of the desert cottontail and gambel's quail, thus its main food supply is only accessible for a restricted time duration every year. however, for this, the gila monsters exhibit three adaptations, which elevate their survival rate in the desert condition.  

first, is that they possess huge bodies, and can consume greater concentrations of food during a small period. secondly, they require to feed infrequently as they can accumulate fat in their tails. ultimately, their rate of metabolism is very low, so that they can burn their reservoirs of fat gradually.