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This is a medical anaesthetic with some psychedelic effects. This is most easily found in small green pressurized gas chargers used to whip cream and available in many large stores. They fit in soda syphons in exactly the same way as the usual CO2 chargers they resemble. This should never be breathed directly (it could freeze the lungs) but always from a balloon and also diluted with air. There is a warning on the box -- 'Do not inhale. Misuse can be dangerous to your health'. In fact it seem to be fairly safe. Classic effects of N2O are a very short (a few minutes) and intense trip. I found it pleasant enough but still a little disappointing - no more than a tingling body high.
Other people consider laughing gas a shortcut to another dimension and speak of near-death experiences.
Laughing gas-trips are harmless, fast and strong but they last only a few minutes.