Grid-forming converters are considered a key technology for electrical grids with a high share of renewable energy. However, there is a lack of a general, robust and verifiable definition of what constitutes this behaviour. Therefore, this paper presents three simple scenarios that enable such a definition. They are based on simple black-box simulations that allow manufacturers to protect their development work. It can be shown that grid forming can be clearly distinguished from grid-following controls by their grid behaviour in the proposed scenarios. Extensive results illustrate this and also provide suggestions to deepen the understanding of grid-forming grid behaviour and its impact on system inertia. These findings should inspire the development of a clear definition and ultimately new requirements for converter systems.