Learning objectives help you nail down precisely what you want your students to learn as a result of the activity or project they are going to do.
It's important that a learning objective be specific and limited to what you are doing in that particular project. If you're building a table, you might WANT your student to "learn carpentry" or even "how to make a table". But that's an awfully general subject, so be specific about what skills or knowledge your student will develop as a result of your project.
Students will be able to make a table.
By building a table, students will be able to correctly use a measuring tape, miter saw, and proper safety equipment.
The second example includes the component skills involved in creating that table. Measurement, sawing, and safety are the attributes of table-building that comprise the learning objective in this example. There are a TON of possible component skills you could teach, but a good learning objective helps you stay on track while teaching a skill. It's specific and limited to
the project at hand. Save your lofty goals for the series of lessons required to fully teach carpentry. For a single project, focus in on the component skills and knowledge someone WILL learn as a result of making it.
To do this with any project, you should answer the following questions:
Whom am I teaching?
What is the project I'd like to incorporate into my lessons?
What are the component skills required to complete the project? (measurement, sawing, safety, comparison with other tables, synthesizing a design, embellishing a design, etc.)
Which of those skills should they learn? (You can draw from state standards, your curricula, what you want your kid to learn before he goes to college, whatever.)
By making _____ (project), students will __________ (verb) __________ (specific skill/knowledge).
By making (a table), students will (understand) (how to join two pieces of wood using dovetail joints).
By building a tabletop trebuchet, students will be able to recognize the design features of a common medieval siege weapon and explain how those features work together to launch a projectile.