How do you got part of the formula to stay and the other part of the formula
to change when using the drag down handle?

For example:
I have
A1=<data B1=12 C1=(A1*B1)

When I pull the handle down currently, I get:
=(A2*B2)
=(A3*B3) etc.

I'd like:
=(A2*B1)
=(A3*B1)
=(A4*B1)

How do I get the B1 to not change?

Thanks in advance for your help!
--Dax