Name one eight-letter word with 'kst' in the middle, 'in' the beginning, 'and' at the end.?

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

What Word am I Looking For ?

With the help clues provided in the image, can you find the 5 digit key?

A man and a dog were going down the street. The man rode, yet walked. What was the dog’s name?

A man is trapped in a room. The room has only two possible exits doors. Through the first door there is a room constructed from magnifying glass. The blazing hot sun instantly fries anything or anyone that enters. Through the second door there is a fire-breathing dragon. How does the man escape?

Dwayne Johnson was running away with the loot from a heist in his car along with Vin Diesel. One tire was punctured and he dropped down to replace it. While changing the wheel, he dropped the four nuts that were holding the wheel and they fell into a drain. Vin Diesel gave him an idea using which they were able to drive till the rendezvous point.

What was the idea ?

If you're 8 feet away from a door and with each move you advance half the distance to the door. How many moves will it take to reach the door?

A bag contains 64 balls of eight different colours. There are eight of each colour (including red). What is the least number you would have to pick, without looking, to be sure of selecting 3 red balls?

What breaks yet never falls, and what falls yet never breaks?

What is both possible and impossible at the same time?