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Triangle or No Triangle?

Age 5 to 7 Challenge Level:
Max from Kiker Elementary School in the United States wrote saying:

This is a square. It has 4 equal sides. It has 4 corners. Each corner is 90 degrees.

Anahita from Guru Gobind Singh Khalsa College in Chigwell told us:

This is a 2-D shape. It has 4 right angles and 4 equal sides. It is known as a square. A right angle is equivalent to 90 degrees.

Nawaf from Pimlico Primary said:

1, 2, 3, 4, 5 are triangles.   6, 7, 8, 9, 10 are not triangles

A triangle has 3 edges and 3 vertices.
For the ones that are not triangles:
One is curved and does not have straight lines
One has a gap in it.
One is not a triangle because it has 4 edges.
The last one is not a triangle because the vertices are not pointy.

Thank you for these solutions. You may be able to find other ways of separating squares from triangles.