A goat is tied to the corner of a barn with a rope. How much grass can it reach? Drag the slider to let out more rope. In front of the barn the goat sweeps three quarters of a circle; where the rope bends around a corner, what is left of it sweeps a quarter circle more.
The source code is at github.com/KirillOsenkov/LiveGeometry. It is written in .NET and Avalonia.