Toy with build-time effects

A design for build-your-own (BYO) toys in which each build participant is provided a toy body or base part and asked to select a data key for their toy. The key has associated with it a set of build-time functions or special effects as well as post build-time functions. The toy body includes a controller that is operable to sense or detect when the key is properly installed or attached to the toy body (e.g., in a key receptacle or interface) and, in response, to read an identifier (ID) (e.g., a static code as may be provided in an RFID tag/chip). The controller may then operate onboard functional elements such as lights and a sound system to provide functions or special effects linked to that key's ID. Each key type may have different special effects associated with it for use during build time and during post build time.