Alex Salmond resorted to reading out a poem from the Cat in the Hat to MPs on Tuesday, as he attacked the possibility that the EU referendum could be held on June 23.
The SNP and parties from Wales and Northern Ireland have said holding the vote at the end of June would overshadow the elections to the devolved parliaments that take place on May 5.
David Cameron has not announced when the referendum will be, however he has strongly hinted June 23 is the preferred date.
Speaking in the Commons today, Salmond said: "We are not trying to get him to name the day we are trying to get him to name when the day is not going to be, it's a question of 'calculatus eliminatus'."
Quoting from Dr. Seuss Cat in the Hat poem, Salmond said: "When you've mislaid a certain something, keep your cool and don't get hot. Calculatus eliminatus is the best friend that you've got.
"Calculatus eliminatus always helps an awful lot. The way to find a missing something is to find out where it's not."