Thinking about playing a trick on my kids with little footprints on the cement porch. Would food coloring wash out, or should I use something else?
You would likely have to use something that is water soluble because cement is porous and will absorb liquids on its surface. If it were me, I wouldn't use food coloring, I'd use water color paint. You could probably find it at a hobby store. If the cement area where you are planning this caper is exposed to the weather, the footprints should disappear after a short time. If they need to disappear more quickly, some diluted bleach would speed the process along. You can also speed the process of making the tracks by making a stamp from a potato. Just carve enough away from the potato to leave a mound that sticks out/up in the shape you want the tracks. You can then just dip the potato into whatever colored mixture you decide on, and quickly stamp your tracks on the cement. That would be quicker than drawing the tracks if you're planning on quite a few tracks. Good luck with the prank!
