When I brush my toddler's teeth, I want good access to the lingual (inside) side of the lower back teeth, to brush them. Thus, I want my kid with parted lips and teeth and with the tongue pulled away from the lower back teeth. When I say "say [a]", that parts the lips and teeth, but the tongue oftentimes remains resting against the lower back teeth. What can I say ("say […]") to get the desired mouth position?
 I think that's the transcription, anyway. The sound I say is the (central?) one I use in dot-com and pot and flop and Tom (not the back [ɑ] in calm or far or con or father). Cf. http://tinyurl.com/qqm8.