Note that you cannot execute a reusable part in isolation. It has to be imbedded in a form before you can observe and test its operation.
To be able to test your reusable part create a simple form like this:
Follow these steps:
Step 2. Create your Reusable Part
Step 3. Use your Reusable Part in your Test Form
Step 4. Add More Functionality to the Reusable Part
By the end of Step 4, you should realise the Benefits of Reusable Parts.