Explain why fossils don't tell us eveything about past plants and animals.

Fossils are the remnants of plants and animals that lived millions of years ago and are now extinct.They are generally formed when the dead remains of plants and animals get covered by layers of silt, sand or calcium carbonate over millions of years.
So, fossils only tell about major details such as their skeleton, shapes and size of the body, etc but  not whole thing about them.
Fossils serve as a great source for studying evolution(slow change ) of living organism. Studying of fossils helps in knowing about the extinct organisms and their relation with the present living forms.