Here is an example of a series of activities that could prompt the children to think about triangles and discover some of their characteristics.
Ask each child to make a triangle with his/her loop and ask several children to talk about their triangle and explain how they know it's a triangle. If responses are very vague - don't try to 'teach' a better answer yet, just move on.
Invite the children to move their fingers to create another triangle and hold it in place. Ask each child to try to find someone else who has made a triangle that looks the same.
Choose some children to explain why the triangles are the same.
Now ask the children to find someone who has a 'different' triangle, and choose some children to explain the differences.
Ask the children to form work pairs and have them make at least five different triangles, drawing each triangle on separate pieces of paper. Invite the pairs to now sort their triangle drawings into two (or more groups). Bring pairs together and have them explain their groupings to each other. Ask the foursomes to now put all their triangles together and agree on a sorting system, which can be displayed and labelled to share with the rest of the class. Finally, return to step one and ask each child to make an triangle, talk about it and explain why it's a triangle. Have they improved?