Not just for coffee! It helps to insulate your cup (hot or cold).
But best of all, it identifies yours amidst the other to-go cups!

Supplies:

Any small amount of worsted weight yarn (about 30 yds).
Size 7 or 8 Knitting needles, either double point or circular, which ever you prefer for knitting in the round (I use two circular). Gauge is about 4.5 sts = 1 inch.