The which switch

Have you noticed how people use "which" to rearrange their thoughts in mid sentence? Today a reporter describing J P Morgan Chase's recent report on its real estate loans said the following:

Then there's jumbo, which we don't know how much of JPM's business is composed of that.

You can convert that into a grammatically correct sentence as follows:

Then there's jumbo; we don't know how much of JPM's business that comprises.

By the time you've said "which," however, it's too late.