$\textbf{Lighting Bulb}$

There are $100$ bulbs, all are off, each of which is controlled by a switch. Joe was playing with them in the following way:

• In the first round, he toggled every switch. So, all the lights are on now.
• In the second round, he toggled switches $2$, $4$, $6$, $\cdots$, $100$. Now half are on and half are off.
• In the third round, he toggled switches $3$, $6$, $9$, $\cdots$, $99$,
• $\cdots$
• In the $10^{th}$ round, he toggled switch $10$, $20$, $\cdots$, $100$
• $\cdots$
• In the $100^{th}$ round, he toggled the switch $100$

Now, the question is, how many bulbs are on at the end?