scissors 是複數還是單數?

我想知道如果要寫(那是一把剪刀)
可以怎麼寫
That is a sissors.還是
That are scissors.
一定要用That is a pair of scissors.嗎?
因為剪刀它本身就是複數型是嗎?

因為你可以說
That is milk. Those are milk.

That is water. or Those are water.

scissors 這個字很妙,本身是複數型 所以有點confused