The stiff backing

Original fiction extract.

Our puppet’s painted face kept bending whenever we lifted it. The drama teacher brought a rigid piece of backing board and laid it behind the paper. It stayed flat even when I held it by one corner. We attached the face to it, leaving the mouth free to move. During rehearsal, the nose no longer folded into the chin. I had wanted to add another coat of paint. The teacher let me paint the back of the new board instead, since the problem had not been a shortage of colour.

Use your own words to explain what the word “rigid” means in this extract.