Difficult

What word is a plural, but when you add an "S" to the end, it becomes singular?

related riddles

Used on a diamond and left on a board. It's great for a gymnast his grip to restore. What is it?

It's usually found dried, but often kept slicker. Above another need, it's said to be thicker. What is it?

What comes once in a minute, twice in a moment, but never in a thousand years?