I always feel that if I have to explain the joke then the strip has failed but in this case I can see the problem for non-native English speakers.

The text on the poster in the third panel is “Free Soft Drinks”. Perhaps it’s a British thing but a soft drink here is — usually — some kind of non-fizzy fruit flavoured drink, in Britain this would be generically be called squash but I guess cordial is a similar thing.

The idea of the strip is that things RMS read in kindergarten/nursery influenced his later life. So we have :

• a standard “it’s good to share” message which seems to be told to all kids of a young age
• an offer of free soft drinks - but obviously the “free soft..” bit is the influence
• a dressing up box from which he pulls a long beard..

Okay it loses it when the joke is written down but that’s why I don’t like explaining jokes :o)

hope this helps anyway
Ryan