Subject: S waves - why don't they travel through liquid?

I don't understand why 'S' waves don't travel through the earth's liquid core 
when other examples of transverse waves we're given in school do; such as water 
waves and light waves (I've read a few of the archive Q's about how water waves 
aren't actually transverse, but I'm still lost, also I've put this as 
nonaligned as I don't need to konw it. I'm just curious, and it might help me 
remember which wave is 'P' and which is 'S', articles I've found just say that 
they won't travel through liquid and that's it - full stop). So anyway, what 
stops 'S' waves travelling through liquid?
Thanks. 

Re: S waves - why don't they travel through liquid?