We know it by measuring the rod when it is cold, and again when it is heated.
Will the rod be larger, as well as longer?
The rod will be larger.
How can we prove this?
The rod, when red-hot, will not go through as small a hole as when it is cold.
How do heat and cold affect most substances?
Heat expands most substances, and cold contracts them.
Do all metals expand alike by heat?
No; some metals expand much more than others.
How does heat expand substances?