Show students the corner of a book and explain to them that this is what a right angle looks like, and that, in mathematical terms, it consists of two lines intersecting at a 90-degree angle. Challenge students to find five other items around the classroom that also have right angles. Then, show them how right angles are present in shapes (squares, rectangles) and how they can use this fact to find the total angle measurement in quadrilaterals.
Teach students how to use a protractor to draw right angles. They should draw a line through the small circle at the bottom of the protractor, mark a point at the small circle, draw a mark at the 90-degree point, and use a ruler to connect the 90-degree point to the small point that you drew. This will help them to cement the way a 90-degree angle looks in their minds.
Use art to reinforce right angle concepts. Create a tesselation pattern using congruent shapes to fill up a piece of paper. Make sure the tesselation pattern, for each student, includes a right angle or two. Give students creative license as to the coloring of the pattern and all aspects of the pattern except for the right angle. Students will enjoy this activity, and you will have some young art with which to decorate your classroom walls.
Take your students outside and find right angles in the real world. For instance, the foundation of the school building is built on right angles. Discuss the purpose that serves, whether it is common to all buildings or unique to your school and the role that it plays in stability of the building. Likewise, the schoolyard's corners likely consist of right angles. Talk to your students about why those planning the school may have made that decision.