You really want to know how to put the words in order after "what". This is the correct sentence: "We all know what rubber is." The words follow a regular order because it's not a question. The question would be, "What is rubber?" The question word order happens at the beginning of a sentence. If your question word ("what") sits in the middle of the sentence, then you use a regular word order. You don't use "We all know what is rubber" (a typical learner mistake), unless you are talking about things made of rubber.