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. I thought up this one while working at the Museum of Science in Boston. 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. Wet some goldenrod paper, then drip some ammonia-based cleaner upon it. Just the ammonia fumes turn the paper red. Wet some goldenrod paper.