Convert \( 90^{\circ} \) to radians without a calculator.

To convert \( 90^{\circ} \) to radians, you can use the conversion factor that \( \pi \) radians is equivalent to \( 180^{\circ} \). Since \( 90^{\circ} \) is half of \( 180^{\circ} \), you can set up the conversion as follows: \[ 90^{\circ} \times \frac{\pi \text{ radians}}{180^{\circ}} = \frac{90\pi}{180} = \frac{\pi}{2} \text{ radians} \] So, \( 90^{\circ} \) is equal to \( \frac{\pi}{2} \) radians.