Here is a link to Edheads page on simple machines. After you check out the activities, tell me which two machines you would want with you if you were stranded on a deserted island. Tell me how you use them. Remember, do not click links off site. Only do the activity on the page.
If you need some extra help, here is an Edhead page with more information, and here is an Mkids page, here is a Museum of Science page.
Here are links to eThemes on simple machines: Beginner, Advanced.