A plastic funnel is used to pour liquids through narrow apertures.
Have you ever tried pouring a drink from a glass back into a bottle ?
If we imagine a funnel as a complete cone (why isn't it?), what shape cone would use the least amount of plastic to manufacture a funnel containing a set volume?
You don't need a specified volume, but it may help to ask what proportion cone would hold a litre of liquid using the least amount of plastic.
See the Hint for more help.