Explanation:
A truss is a structure made up of a series of connected members that form triangular units. These triangular units are called triangles, and they are the basic building blocks of a truss. Therefore, it is correct to say that simple trusses consist entirely of triangles.

Reasons:
1. Definition of a truss: A truss is a structural framework composed of triangular units connected at their ends. These triangular units are rigid and do not change shape when forces are applied to them.
2. Simple trusses: Simple trusses are trusses that are statically determinate, meaning that they can be analyzed using the equations of statics alone. These trusses are typically made up of straight members connected at their ends to form triangles.
3. Triangle as a stable shape: Triangles are inherently stable shapes. The sides of a triangle do not change length when forces are applied to the joints, and the angles between the sides remain constant. This stability makes triangles ideal for use in truss structures.
4. Load distribution: In a truss, the loads applied to the structure are transferred through the members to the supports. The triangular shape of the truss members ensures that the loads are evenly distributed among the members, resulting in an efficient and balanced load distribution.
5. Tension and compression forces: In a truss, the members are subjected to either tension or compression forces. Triangular shapes are particularly effective at resisting both tension and compression forces, making them well-suited for truss structures.

Conclusion:
In conclusion, simple trusses consist entirely of triangles. The triangular shape of the truss members provides stability, efficient load distribution, and resistance to tension and compression forces.