Ask your victim to hold out their palm.
"There's your house," you say, pointing at the centre of their palm.
"There's the garden," you continue, pointing slightly to the left/right of centre
"Where do you want the fishpond?" you ask.
Your victim will then point somewhere else on their palm, and you, in response, will cough up a massive great greeny, and, with unnerving accuracy, place the "fishpond" at their chosen location.

A friend of mine had a variation on that trick, but with less build up and panache:
Greeny cocked and ready he would get a girl to open her mouth by simply saying 'Open wide, go on. Nothing'll happen...' and saying 'trust me' repeatedly, coupled with pleas of 'come on, why don't you trust me?'.
Then phutt - a rolled-over tongue shot ripe green straight in. Not everyone was so trusting, and it often took considerable leg work to secure a victim, sometimes more than one day.
It wouldn't have surprised anyone if he'd gone through a romantic montage with a girl, ice-skating and running along beaches, simply to shoot some snot into a girl.