21

Number
$21$ (twenty-one) is:


 * $3 \times 7$


 * The $6$th triangular number, after $1, 3, 6, 10, 15$:
 * $21 = 1 + 2 + 3 + 4 + 5 + 6 = \dfrac {6 \times \left({6 + 1}\right)} 2$


 * Thus $21$ is the number of pips on a die.