I was thinking about that when I was dropping my 6 year old off at some hobbies earlier - it’s pretty much expected to have learned how to ride a bicycle before starting school, and it massively expands the area you can go to by yourself. When she went to school by bicycle she can easily make a detour via a shop to spend some pocket money before coming home, while by foot that’d be rather time consuming.
Quite a lot of friends from outside of Europe either can’t ride a bicycle, or were learning it as adult after moving here, though.
edit: the high number of replies mentioning “swimming” made me realize that I had that filed as a basic skill pretty much everybody has - probably due to swimming lessons being a mandatory part of school education here.
It’s not really transactional. It’s just a situation where you got left out of the birthday and happened to go out to the same place where the birthday is being celebrated. However, it’s interesting to note that there is no such thing as a surprise birthday party. The birthday boy or girl is the one that throws the party because of the reciprocity aspect. You wouldn’t be caught dead attending a birthday without a present for the person whose birthday it is. You also wouldn’t be caught dead letting people bring you birthday presents AND buying you dinner. It’s more like “tit for tat” than “transactional.”