Laughter is said to be the best medicine.
Well, according to this study, laughing is good for your abs,
even if this has only been demonstrated when laughter is obtained in some way artificial means:
laughter yoga, a form of yoga that uses self-induced laughter.

Social, psychological and physiological studies have shown that laughter requires strain on the trunk muscles.
The purpose of the study was to quantify the activation of the trunk muscles during "laughter yoga",
compared to lifting and flexing exercises.

Muscle activity during laughter yoga exercises was measured by surface electromyography on 5 muscles of the trunk:
the level of activation of the internal oblique muscle during laughter yoga would be even higher than in traditional exercises.
The multifido, sacrospinal and rectus abdominal muscles were activated, but to a lesser extent, during "yoga"
while the activation of the external oblique muscle was comparable with the exercises.
The results therefore indicate that laughter has a positive effect on muscle activation of the trunk,
although the researchers point out that further research is needed to understand if it is a good exercise also to improve the stability of the spine.