Powerpoint suitable for KS2 and KS3 Maths: Shape, space and Measure / Geometry - Triangle properties. This animation shows how angles in a triangle lie and a straight line and therefore add up to 180.
I usually get students to test this out with their own card triangles and use this animations as a summary.
I like this as it shows clearly how and why the angles in a triangle add up to 180 degrees. You could get the pupils to do this with paper too. Nice to start off a lesson on angles showing this and starting a discussion.