Here's my BEST demo with this. Tell the kids, lie, that you just found out about a medical breakthrough! It is a solution that, when poured onto your palms, forces the blood to capilate up thru the pores and become available for blood testing, thus eliminating that painful needle!
I have a few pieces of goldenrod paper taped to the blackboard and, while describing the above "story", lie, I stick my hand in a beaker of ammonia. Immediately walk up to the goldenrod paper on the board and "SLAP" it hard with your open palm! You will immediately hear the "Ewww" and "Oh, NO!" when the kids see a blood splattered handprint on the paper!
I also, when I'm feeling particularly cruel, fake cutting a finger or something while the kids aren't paying attention and screaming, "Ouch! I cut myself!" loudly while swinging my hand, previously soaked in ammonia, toward a piece of goldenrod on the desk or taped on the wall! The resulting splatter of crimson colored "blood" as the ammonia drops strike the paper is a great effect on Halloween!
How do it work? If alkaline solutions contact the paper, it reacts RED! Spray it with Windex, it contains ammonia, and it instantly turns bright red! Astrobrights Galaxy Gold paper is an acid/base indicator strip. Dip it in a base solution (like ammonia cleaner, baking soda in water, etc.) and it turns bright red. Dip it in acid (vinegar, lemon juice, etc.) AFTER it turns RED and it turns yellow again.
Troubleshooting Note: It seems that some kinds of goldenrod don't work! The "good" kind is Astrobrights Galaxy Gold WAAB57A, from Wausau Papers. Goldenrod paper is available at Staples and Office Max. Do yourself a favor, tho. Take some baking soda to the stationery store, so you can test the paper (the "good" kind will turn red when moist baking soda is rubbed on it.) I found the following addititional demos at the Science Hobbyist site by William Beatty.
Cut it into strips, dip it in acid or base. It turns colors. duh. Dip it in base so it turns red, then dry it out. This gives you an acid-indicating paper which starts out red and turns yellow in acid. Put dilute vinegar in one jar, baking-soda solution in another. (Baking soda dissolves better in warm water.) Use paintbrushes to paint on the goldenrod. Baking soda solution turns the yellow paper red. The vinegar solution turns previously-reddened paper yellow. Paint an invisible picture with vinegar on yellow goldenrod, let it dry, then spray it with baking soda solution. It turns red everywhere except the places having vinegar. Draw "invisible" patterns or messages on the paper with rubber cement, diluted Elmer's glue, transparent tape, etc., then spray it with alkaline solution. The paper turns red except where your drawing has sealed it. Yellow artworks on red background appears. Wet a strip of previously-reddened goldenrod, then lower the strip into a half-full glass of carbonated beverage. Don't let the strip touch the liquid. The strip turns orange as the transparent pool of carbon dioxide forms carbonic acid in the wet paper. This lets you "see" the invisible pool of CO2 gas which fills the cup. (Only works in a draft-free room, where the CO2 gas remains atop the cola.) Dust your hands with baking soda. Claim that you have "Alien DNA Test Paper," and if it turns red, it indicates that you are not human. Have your audience put their thumbprints on some wet Goldenrod paper. Anyone with baking soda on their fingers (you!) will leave a thumbprint which slowly turns red.
ELECTROLYSIS: wet some goldenrod paper with salt water and place it on a sheet of aluminum foil. Use clipleads to connect the positive terminal of a 6v or 9v battery to the foil. Connect a wire to the negative battery terminal. Now drag the negative wire across the wet goldenrod, and it turns red. Write with electrochemistry! If you reverse the polarity of the battery, you can erase your red drawings. If you replace the goldenrod with previously-reddened paper, the reversed battery connections let you draw in yellow on a red background. (the positive plate creates acidic solution, while the negative plate makes alkaline.)
MY OLD ACID RAIN DEMO: Wet the inside of a glass jar. Light a match, blow it out, then collect the smoke inside the upside-down jar. After awhile the drops of water collect nasty combustion products from the smoke and become acidic. Touch the drops to previously reddened goldenrod paper, and it turns yellow, indicating acid. Instant acid rain! And might you think twice about smoking cigarettes and putting acid in your lungs? I thought up this one while working at the Museum of Science in Boston.
NOTE: Young kids shouldn't perform the following demonstrations. Ammonia is somewhat toxic, is nasty if inhaled, and is dangerous if splashed in eyes. Adult supervision only. Wear safety goggles.
AMMONIA DEMOS: Wet some goldenrod paper, then drip some ammonia-based cleaner upon it. Notice that the red drops have red haloes around them? Just the ammonia fumes alone can turn the paper red. Wet some goldenrod paper. Put some ice cubes in a jar, then pour in a little ammonia and wait for some cold ammonia gas to build up. Carefully pour the transparent ammonia gas over the wet goldenrod paper, and it flares red. Dip a wet strip of un-reddened goldenrod into the seemingly-empty jar, and you'll discover the depth of the pool of cold ammonia gas. Make wet marks on dry goldenrod, and when cold ammonia gas is poured over it, the wet marks turn red. (Don't leave the jar of ice cubes sitting around, dump it out so that passersby can't take a sip from your glass of "icewater.") Use a smoke-ring box to shoot invisible ammonia "smoke rings" at wet goldenrod paper. Little red puffs appear where they hit. Freak out Kinko's Copies employees by buying one sheet of goldenrod, asking for the bottle of glass cleaner, then yelling "look!" while spraying the paper with the ammonia-based cleaner. But be warned, I've been doing this for awhile, so the secret is spreading from Kinko's to Kinko's like a mind-virus. They may already know about it. The Goldenrod Paper Secret was passed on to me around 1987 by Dr. Roy Gould of Harvard CFA, who got it from his brother, an R&D chemist in NYC, who heard about it as the "secret" traveled from chem lab to chem lab across the country. I wrote up this paper and posted it on the web around 1995. Since then it has spread all over the place!