I have a form which pops up inside a lightbox via a link.

Within the form there is a field which needs explaning to the user, so I want to have the explanation pop up inside another lightbox.

After endless hours of googling, Im yet to find a solution that allows for this.

Im currently using thickbox as my first lightbox, but how can I add a second form of lightbox so that the field explanation (accessed by clicking a "What is this?" link) opens in a second lightbox?

So im hoping to achieve:
link > lightbox with form > 'what is this link' > 2nd lightbox with explanation

Id be really grateful for any ideas!