I have been studying stopping times, however i am stuck on how to prove that a random variable is actually a stopping time even if it is intuitivly obvious. For example, How do i show that $\displaystyle \alpha=inf\{t>0 : B_t=-1 or B_t=2\}$ is a stopping time w.r.t the natural filtartion $\displaystyle F_t^B $ generated by $\displaystyle B_t$....

I know that i must show $\displaystyle \{\alpha<t\}\in F_t^B$ but dont actually know how to do this....