Although the presentation made in the blog of scientific visualizations esquemat.es is a very graphic demonstration of how the angles of a triangle always add up to 180 degrees, I like the version that demonstrates it using the properties of the secant lines and angles determined by two parallel lines and one secant.
I encourage all parents to try to explain to the children these concepts of Euclidean geometry tremendously easy and very intuitive and that surely your children will never forget. And to show that the three angles of a triangle total 180º I recommend taking a look at the video below that explains it quickly and also makes use of good graphics to illustrate it. And is that one of the characteristics of mathematics is encourage imagining to solve problems in a broader context so the sooner you begin to see the kids how they can apply concepts learned much better. Because children with ages close to ten years have ample capacity to understand the statement, discuss the approach to make relating concepts and execute the solution in an elegant and clean way.
In any case, in Microsiervos we also recommend taking a look at Learn at Mathematics Realm to continue learning and practicing with geometry.
To bring the geometry to children and see that what they have around is full of lines, parallel, secant and anglesI recommend taking a tour of a city and taking a look at the buildings that are everywhere. There you can see right angles, triangles, circumferences, secants and 180º and 360º angles. They are very simple concepts that are learned quickly and for which the children's ability is prepared to understand them and put them into practice.
Also books like that of Juan Medina and Fernando Blasco they can help to initiate the children, with the help of their parents, in the abstract ideas that are applied in mathematics and that help to shape the critical vision and face the problems in a rational way.
