The problem most creationists have with the Theory of Evolution (herein referred to as Evolution) is that it is taught as fact. Fact implies proof. Yet when proof is requested, the typical evolutionist points to the fossil record and says that from these we can "reasonably conclude" that all life has evolved from a common ancestor.
Reasonable conclusions do not constitute fact. Further, an unbiased look at the fossil record reveals no evidence that Evolution is a reasonable conclusion to draw at all. Given that the currently accepted evolutionary time line is correct (which is in itself a debateable issue), the thousands of fossils found to date span millions of years yet reveal no intermediate life forms. That is, bats have always been bats, frogs have always been frogs, fish have always been fish, and man has always been man. Certainly there have been alterations within species, but never a shift to a different animal.
Bats are a good example. For bats to have evolved, they either came from birds-turned-mammal or from mammals that grew wings. Either way would have required several million years of mutation as bats became bats. Thus, we should easily be able to find fossils from some of the billions of individuals that lived and died on that long trail to bat-hood. Yet not a single one has as yet been uncovered. There are fossils of bats and fossils of the alleged ancestors of bats, but nothing in between.