I believed that the numbers 0 to 100 either were a tower or lived in a tower (it was a bit vague), with each 10 numbers having a floor each. The numbers at the bottom, the single digit numbers, always wanted to get to the top, to defeat the evil 100 and his henchman 99. So they would work together to get there, through multiplication. For example 6 would multiply with 7 to get to the 40s level at 42, which was a rather dark and dingy place with a low ceiling. At this point 7 would thank 6 and multiply himself by himself to get to 49, from which he could pull himself up to the 50s floor, a grand hall with marble columns. For some reason I pictured him and 8 then having a cup of tea there. It got weirder with some of the other numbers though. I imagined 5 as the real hero of the group, and for him was the greatest challenge - the 80s floor was a harem of seductive numbers, all throwing themselves at him, tempting him to stay. But 5 would steadfastly go on, pushing past up to the 90s floor where he would confront 100 and save the day. At this point the correspondence to real mathematics was gone and it was just a story in my head. I've never really been any good at maths since then...
A childhood belief about classes

others believed things like this
- I used to believe that numbers had gender, that the odd numbers were boys and even numbers were girls. — Evelyn
- I believed that numbers eventually just ended. That they stopped somewhere after 1000.— Jamis
- Numbers went up to 100 then restarted at zero.Anon
- The highest of all numbers was "a finnety."— Becky
- i used to belive that 1 to 10 were the only numbers and all others were made up.....— Eric
- I used to believe there was a limit on the numbers you could count — Carmen