Toy building unit

A building unit of a toy building set, the building unit comprising:

• at least two sections, wherein each section is connected to another section with at least one hinge, wherein a number of sections is equal to one plus a number of hinges, wherein the building unit is-configured to fold over multiple cycles at the at least one hinge from a flat position into a at least one open position or a closed position, wherein the closed position is a three-dimensional position with a hollow interior;
• wherein the hinges are configured as a groove on one surface of the building unit in the flat position and as ridges on an opposite surface of the building unit in the flat position;
• wherein at least two joins integral to outer edges of the building unit in the flat position are configured to be brought together in pairs to form edges of the building unit in the closed position;
• wherein the at least two joins have one or more integral teeth;
• wherein the at least two sections, the at least one hinge, the at least two joins, and the one or more integral teeth are one piece;
• wherein the one or more integral teeth of paired joins are configured to interleave to form a closed edge of the building unit in the closed position such that friction between surfaces of one or more interleaved teeth holds the building unit together in the closed position;
• wherein a friction fit edge is configured to unpair by a pulling apart one or more interleaved teeth;
• and
• wherein in all positions of the building unit, the one or more integral teeth are configured to retain a fixed orientation relative to their integral join and their integral section.