Tickling is a phenomenon. Outwardly a playful, laughter-inducing activity, it can be lighthearted and pleasant. But it has a darker side: tickling indicates dominance, can reduce someone to tears, and it has even been used as a method to kill people at various times in history, including the Holocaust.

So, what is tickling? First, it is necessary to note that not all tickling is the same. There are two types of tickle sensations that are perceptible to human touch. The first is called knismes...
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