The triangle is a three-sided polygon. Introduce the lesson on triangles with a snack of Doritos. Pupils will notice the triangle shape of each chip. Draw some shapes on the board and ask pupils to identify the triangles. Give each pupil a copy of the shapes on the board; have them cross out the ones that aren't triangles and trace the triangles. Review and discuss the correct responses.
Triangles have first and last names. Draw and label an example of each type of triangle on the board. The equilateral triangle has three sides of equal length. An isosceles triangle has only two sides of equal length. With a scalene triangle, each side is a different length. Look again at the handout and help pupils identify and label the types of triangles. Touch your thumbs and index fingers together to create a triangular shape to peer through. Have the pupils hold their hands in the same way, then look around the room to find more triangles. They should name the triangles as they find them.
During physical education time, appoint three pupils to be "corners" by taping a white triangle on the front of their shirts. Show them how to extend their arms to be the corners of a triangle. Color-code each side of the human triangle by attaching colored triangles to the other pupils. When the pupils hold hands, they become one big triangle. Help them exchange colors and arrange themselves in different numbers to make equilateral, isosceles and scalene triangles.
Write the words triangle, tripod and tricycle on the board. Ask the children what the words have in common. Note that "tri" means three. Point out that each item has three key parts -- sides, legs or wheels. On a table at the front of the room, position a tripod on a large piece of blank paper. Ask the pupils to come up in small groups to draw straight lines from one leg of the tripod to the other. What type of triangle is formed on the blank paper? Attach a camera to the tripod and take a picture of the class making triangle shapes with their hands and peering at the camera. Print and post the photograph in the classroom.