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New Scientist offers the definitive guide to everything you wanted to know about being green but were too embarrassed to ask

If I switch the light on and off every time I enter and leave a room, does this use more energy than leaving it on all evening?

Switching the light on and off does saves energy, but there is a catch. Every time you flip the switch, the bulb takes a jolt of electricity, which shortens its life. Studies by the Electric Power Research Institute in Palo Alto, California, found that turning low-energy compact fluorescent bulbs on and off at frequent intervals can shorten their lifespan by as much as 75 per cent. The institute’s director of energy utilisation, Tom Reddoch, suggests leaving energy-saving bulbs on if you will be out of the room for less than 15 minutes.

How clean does the pizza box have to be for it to be recyclable? Likewise cans and bottles

According to the Waste and Resources Action Programme (WRAP), based in Banbury, UK, pizza boxes are often not recyclable. That’s because grease from the toppings contaminates the cardboard, making it useless to paper mills – though it can still be composted. Such impregnation is not a problem when it comes to cans and bottles. Nevertheless, they should be rinsed to remove food remnants so as not to attract vermin. Plastic should also be clean, and lids removed from bottles so they can be squashed flat. WRAP recommends rinsing waste items in old washing-up water to save energy.

Are laminated juice cartons recyclable?

Yes – but …