Why don't alkynes exhibit optical or geometric isomerism?

Alkynes always form a triple bond. This being so, essentially, all you have are the two carbon molecules and the triple bond! No matter what way your turn it, or how you look at it, even if in a mirror (ie. optical isomerism) you will always have the same looking molecule while to have a geometrical isomer cis or trans form should be there.

gen equation for a compound to be a geometrica isomer:

1.YXC = CXY

2.YXC = CXZ