Show that for every natural number n there is 0<p<1, s.t. given two coins, one fair and one with heads probability p, the uniform probability on 1,...,n may be generated by the two coins. Namely - you may choose an element from 1,....,n with probability 1/n.
Once you are done - try to do the same with only one coin (thanks to Ron Peretz for reminding me that this is possible)!