Show your audience a small, green ball.
Then cover the ball with a handkerchief and have a volunteer from the audience hold it.
Instruct the volunteer to hold the ball with the handkerchief over a clear cup.
The volunteer then releases the ball into the cup.
You remove the handkerchief from the cup, and what do you see? Not a green ball, but a red one!
How did that happen? At the start you had two balls, identical in size. The red ball was simply hidden in your hand.
At the right moment you switch the balls. And then while everyone is watching the volunteer, you drop the green ball into your lap or pocket.
What could be simpler than that?