When two coins are tossed, the sample space is:
\[
S = \{\text{HH, HT, TH, TT}\}.
\]
The event of getting both tails is \( E = \{\text{TT}\} \).
The probability is given by:
\[
P(E) = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}} = \frac{1}{4}.
\]