To find the probability of Leah rolling a 6 on a six-sided number cube two times in a row, we can break down the probability of each individual roll and then combine them.
The probability of rolling a 6 on one roll of a six-sided die is:
\[ P(\text{rolling a 6}) = \frac{1}{6} \]
Since the rolls are independent events, the probability of rolling a 6 two times in a row is:
\[ P(\text{rolling a 6 twice}) = P(\text{rolling a 6}) \times P(\text{rolling a 6}) = \frac{1}{6} \times \frac{1}{6} = \frac{1}{36} \]
Therefore, the probability that Leah rolls a 6 two times in a row is:
\[ \frac{1}{36} \]
Thus, the correct answer is \(\frac{1}{36}\).