Lasso

draw a loop out of your green and back. a calf snaps an open rope.

draw a rope out of your green patch and back into it: everything the loop encloses becomes yours in one fill, but a calf that trots over the open rope snaps it

Put your finger on your green, draw out across the sand and come back: everything the loop closes off turns green at once, with any calf caught inside. While the rope is open a calf touching it snaps it, and that rope is gone. Each ranch has a goal and a few ropes; a bigger loop takes more and stays open longer.

Concepts: area, risk.

An interactive you play with one thumb, on Wundr.