Magic Flower Seeds
The magician shows a can of "Uncle Don's Magic Flower Seeds" and tells the kids that they grow flowers almost instantly. He picks up a small pot and dumps some seeds into it. At this time in the routine, you can use any of the comedy wands on the market to try to get the flowers to grow, but nothing happens.
You explain that you forgot to water the seeds, so you pick up a small cup of a very special water and pour some in. You explain that this is "spring water" and when you turn the cup over, a spring slinky fall out. After waiting a moment with nothing happening, you look at the can label and notice it says "Grows in the Dark".
You place the can over the pot and get the kids to yell the magic words, and when you lift the pot, a cute flower bush appears that is bigger than the can!