Tickling for too long during one session can cause feet to lose some of their sensitivity. My girl has ticklish feet especially during the first few minutes of a tickling episode. When I first start, she has a strong temptation to clench her feet to get some relief from the tickling, but I make her keep her toes curled back. I love to see the tension in her toes as she longs to clench but doesn't because she knows that I like for her toes to stay curled back while I tickle them. However, she can't take an electric toohbrush between her toes. I have to let her move her foot and clench some because that is the only way that she can take it. Her toes never become de-sensitized to the electric toothbrush. If I tickle her too much in one day, she eventually tells me that her feet are 'all tickled out' and that they need a rest to re-charge. But, there is always tomorrow.