The theory of evolution is really made up of two key concepts, decent with modification (1) and the theory of natural selection (2). Now for the simple arguments in its favor:
1) We have observed it happening in the small scale with selective breeding. For instance, some cows are born with the trait of producing more milk than others. We select those cows and breed them rather than those that don't produce as much milk. After a few generations the average amount of milk produced by the cow population has increased. The only difference is that the selection of the cows is not done through natural methods, but it is easy to see how certain traits would make it more or less likely for an organism to reproduce.
2) We have a long fossil record of relatively gradual modification from a common ancestor for all known life. This is more obvious for more closely related animals (human and monkey, horse and cow).
3) The theory of evolution is the core organizing principle behind modern biological theory and study. The fact that they have a biology textbook in front of them and not a Bible is a potent argument in and of itself.
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