Double Secret Keepers?

[catherine at cator-manor.demon.co.uk]

No: HPFGUIDX 26790

--- In HPforGrownups at y..., s_luhtanen at h... wrote:
> --- In HPforGrownups at y..., catherine at c... wrote:
> > Just listening to Fudge's account of what happened prior to the 
> > Potters being murdered, and I heard that Sirius Black was 
planning 
> on 
> > going into hiding.  I thought struck me.  If Sirius was 
originally 
> > going to be their Secret Keeper, then why couldn't James or Lily 
be 
> > his Secret Keeper?  That way all of them would have been 
impossible 
> > to find.
> > 
> > Why didn't they think of that?
> > 
> > Catherine
> > (BTW:  I know the answer is that otherwise most of the plot in 
PoA 
> > would be redundant, but still...)
> 
> Couple of reasons: a) Because the Spell itself makes it impossible.

Excuse me for being incredibly dense, but why does the spell itself 
make it impossible?

Catherine