[C-hemCards]

The answer to this question is 'acidity of terminal alkynes'.

Alkynes have sp hybridized carbons, i.e., more s character than sp2 or sp3 carbon atoms. That high level of s character enables excellent overlap with the s orbital of the hydrogen atom, making a highly covalent bond. This, in turn, polarizes the C-H bond toward the carbon atom more than in sp2 or sp3 bonds, resulting in the acidity.