Final answer:
The easiest way to distinguish a bird's egg from a reptile's egg is by the outer shell, with bird eggs having a hard, brittle calcium carbonate shell and reptile eggs having a more leathery and pliable exterior.
Step-by-step explanation:
The easiest way to distinguish a bird's egg from a reptile's egg is by the outer shell. Bird eggs have a hard, brittle shell made of calcium carbonate, while reptile eggs tend to have a more leathery and pliable exterior. This hard calcium covering of bird eggs and the hard leathery exterior of reptile eggs are crucial for providing protection to the growing embryo. Moreover, it's important to note that the nourishment within both types of eggs is provided by the yolk, and not by endosperm or placenta, which are not present in eggs.