Valid statements can be used to build logical arguments.
'If-then' is a common form of argument. In senior mathematics classes, the notation used for this argument is \(p \rightarrow q\). You can read more about this notation and watch a video from the Math Planet website at http://www.mathplanet.com/education/geometry/proof/if-then-statement.
However, much younger students should learn this form of argument.
A typical if-then problem and variations on it make a good starting point. You can download Insects and Spiders: An Avenue for If-then Thinking.