Question

We should never connect both the terminal of a cell without connecting an appliance in a circuit. Why?

Answer 1

That is called a "Short circuit"

Basically charges enter and leaves the battery in a cycle, this flow of charges is known as current.

This results in excessive current flow in the power source through the 'short,'  a high current will flow, causing the delivery of a large amount of energy in a short period of time.and may even cause the power source to be destroyed.

Answer 2

Answer:

The two terminals of the electric cell are never joined together without connecting them through a switch and a device like a bulb because if we do so the chemicals in the electric cell get used up very fast and the cell stops working.