Mark LaValla, age 9, of Utica, N.Y., for his question:

WHAT MAKES PEOPLE TICKLISH?

Some people laugh when they are tickled, and others are not affected. It all depends on the 'sensitivity of the skin: those with sensitive skin fall into gales of giggles when a tickling finger starts to probe.

The laughing of the person at the receiving end of the tickle is actually a cry of distress  a begging to the person to stop tickling. When we tickle ourselves, we are in no distress, as we are not at the mercy of someone else and we can stop any time we wish. And, as a result, we do not laugh when the tickling is self imposed.