Re: WHAT SHAPE IS THE TUBE USED BY A STRING LEVEL?

You were close to being on track but you discovered why a straight tube 
doesn't work.

The bubble in the liquid (any liquid should work, maybe with a little 
detergent added to help create the bubble?) tries to rise to the top of the 
liquid.  When you have a straight tube which is not level,  "top" is the 
end that is higher.  But when the tube is level, "top" is all along the 
tube, which is not helpful.

A level contains a curved tube.  The degree of curve is probably not that 
important.  The more curve, the more you are able to distinguish between 
"almost level" (bubble not centered) and level.

Not sure how to help you other than to suggest getting one of those straws 
that bend with a flexible, ribbed section in the middle.  You can get a 
small curved section, but it won't be very long and you can't see through 
it very well.  It's possible to create a curved tube of glass, but that 
requires bending it in a flame, which is something you would need an 
experienced adult to help create.