I need your help!!! What/That/Which

[anonymousbrainsnatcher]

I don't normally have trouble with spelling and grammar. However, there is something my children say and I am genuinely unsure on how to correct them correctly (IYSWIM!)

They both say:

"I like the one WHAT is blue".

Should it be "I like the one THAT is blue" or "I like the one WHICH is blue"?

tell them it should be 'I like the blue one'

How about "I like the blue one"? But apart from that, my understanding is that it should be "that" in this context, as the information is used to identify the thing in question. If it was supplementary information it would be "which". So:

"I like the one that is blue."

"The one that I like, which is blue, is not the one I'm going to get."

Unfortunately this ruins the old joke, "Why should you never go hungry in the desert?" - (Because of the sand which is there). It should of course be "the sand that is there".

that then.
thanks. I figured it probably was, but got confused because "which" didn't sound completely wrong. Usually these things just sounds right or wrong, but I was stumped here.

Thanks.