Pretty much anything with a tapered, smooth end like that is going to have that kind of an effect, especially with a slick substance applied beforehand. Some hobby based slingshots have dual prongs on them like that with the tips coated in a soft compound plastic that are rounded on the ends and not much bigger that the end of a paintbrush. The coolest thing about one of these is that you could get both soles at the same time or tickle someone's thighs who are wearing tight jeans. Also, some people like to use a ballpoint pen. Just be careful though and don't use too much pressure or it can be painful. I would default to a medium or even a wide tip just to be on the safe side and when you feel like you're ready, move up to a fine tipped one. Mild pressure with a fine tip can produce nearly unbearable tickling and slightly harder pressure can tickle and hurt at the same time for all you sadists out there.