Which word does the below Plexor means ?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

A sundial has the fewest moving parts of any timepiece. Which has the most?

A mile-long train is moving at sixty miles an hour when it reaches a mile-long tunnel. How long does it take the entire train to pass through the tunnel?

A doctor and a boy were fishing. The boy was the doctor’s son, but the doctor was not the boy’s father. Who was the doctor?

Two brothers were watching a horror film on video late one night. One brother dozed off and dreamed that he was being chased by the crazy man from the movie, who was trying to kill him. In the dream, he hid in a cupboard. There was no sound except his heart pounding, and he had no idea where his crazed captor was. He was terrified! At that moment, the video finished, and his brother put his hand on the shoulder of his sleeping sibling to wake him. The shock at that tense moment was enough that the sleeping brother suffered a massive heart attack and died instantly. True or false?

Sherlock Holmes was challenged to make a pre-drawn line smaller without erasing it. He did it in fractions of seconds. How?

In which direction should the missing arrow point?

The product of three consecutive numbers is 7980.
Then the sum of these consecutive numbers would be?

What country can make you shiver?