To calculate the 10% sales tax on a $12.00 purchase, you multiply the purchase amount by the sales tax rate (0.10):
\[ \text{Sales Tax} = \text{Purchase Amount} \times \text{Sales Tax Rate} = 12.00 \times 0.10 = 1.20 \]
So, the sales tax on a $12.00 purchase would be $1.20.