Answer
Osmosis causes limp celery to become stiffer and the skin on your fingertips to become wrinkled in tap water.
Work Step by Step
Limp celery and the skin on our fingers tips have a much higher osmolarity compared to the plain tap water. Plain tap water is a hypotonic solution, meaning that it has a lower amount of solutes or lower osmolarity compared to what is soaking in it. Water moves to the limp celery and the skin to equalize the different osmolarities. The celery will become stiffer as its cells take on more water, or become more turgid. As the skin cells take on water, they will become more turgid, too or swell. Your fingers don't change in size, so only the skin bunches up to form the wrinkled look.
