"...so that the middle does not fall out"

A student is working a problem on the board.

Me: Now what do we need to do?
Student: Collect the like terms!
Me: So can you put the 2x and the 8 together?
Student: No!
Me: Why?
Student: Because they are not like terms.
Another student: You can only collect the like terms. Can a school girl sleep on the same bed as a school boy? Can you mix salt and sugar?

Later, the student forgets to close a set of brackets. Several students point this out, and one explains, "We close the brackets so that the middle does not fall out."