Which word in the English language, beginning with "P" is singular until an "S" is added to the end and it becomes a plural, but reverts back to a singlular word if another letter "S" is added to the end?

How would you combine one-third of 12 and four-fifths of 7 to get 11?

One night Hugh was at work at the recycling plant when he received an unexpected telephone call from a neighbor, listened to what the neighbor said, slammed down the phone and left work and drove straight home. On entering his house Hugh crept upstairs and found his wife in bed with a stranger. He wasn't angry but he was surprised. In fact he crept back down stairs and made his wife some breakfast, which he took up for her to eat in bed.

Why ...

One day Jerry Mayor from Smelliville was visiting the neighbouring country of Madadia and was sitting in the Penistone mainline railway station. He was sitting very uncomfortably in the waiting room and was staring intently at two doors that were at the opposite end of the room. Jerry Mayor was not a happy man at that point. In fact he was in dire distress. He then noticed a young man enter one of the doors. Instantly Jerry, with a great smile...