If you are under the full retirement age and decide to work while you are on social security you could pay pay additional tax. There are 3 scenarios for working while receiving benifits:
If you are the full retirement age of 66 before 2010: If you work and are full retirement age or older, you may keep all of your benifits, no matter how much you earn.
If you reach full retirement age during 2010: we must deduct $1 from your benefits for each $3 you earn above $37,680 until the month you reach full retirement age.
If you are younger than full retirement age during all of 2010: we must deduct $1 from your benefits for each $2 you earned above $14,160.