der Spaß
fun
A feeling of joy, pleasure, or cheerfulness.
Die Kinder hatten großen Spaß beim Spielen im Schnee.
The children had a lot of fun playing in the snow.

fun / joke
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fun
A feeling of joy, pleasure, or cheerfulness.
Die Kinder hatten großen Spaß beim Spielen im Schnee.
The children had a lot of fun playing in the snow.

There are two basic sentence patterns: 'Spaß haben' (to have fun) describes that a person actively feels joy. 'Spaß machen' (to be fun) describes that a thing or activity is enjoyable for a person (in the dative).
Ich habe Spaß an der deutschen Sprache.
I have fun with the German language.
In its general meaning as joy or pleasure, 'Spaß' (fun) is very often used without an article, similar to other abstract nouns (e.g. Glück, Liebe, Zeit).
Wir hatten gestern Abend richtig viel Spaß.
We had a really great time last night.
To express that something is not meant seriously, you use the expressions 'aus Spaß' (for fun) or 'nur zum Spaß' (just for fun).
Ich habe das nur zum Spaß gesagt, sei nicht böse.
I only said that as a joke; don't be mad.
'Viel Spaß!' (Have fun!) is a fixed expression used like a greeting to wish someone a good time during an upcoming activity.
Du gehst ins Kino? Viel Spaß!
You're going to the movies? Have fun!
The expression 'Spaß beiseite' (joking aside) introduces a change of topic. It signals that you want to move from a joking or light topic to something serious.
Haha, sehr witzig. Aber Spaß beiseite, wir müssen über den Plan für morgen reden.
Haha, very funny. But joking aside, we need to talk about the plan for tomorrow.
joy
'der Spaß' and 'die Freude' both describe a positive feeling. 'Spaß' often sounds lighter and fits well with games, free time, funny situations, or enjoyable activities. 'Freude' is usually a bit deeper or calmer and fits well when someone is truly happy or satisfied. You can say 'Spaß an etwas haben' (to have fun doing something) and 'Freude an etwas haben' (to enjoy something), but 'Spaß' feels more casual, while 'Freude' feels somewhat more serious. Learners may confuse the words because both are positive. As a guide: for something funny or entertaining, 'Spaß' is more natural; for real happiness or inner satisfaction, 'Freude' is more likely.

Trotz des langen Schultags hatten die Kinder im Sportkurs viel Spaß.
Despite the long school day, the children had a lot of fun in the sports class.
In der Bibliothek findet Maria ein altes Foto und spürt große Freude.
In the library, Maria finds an old photo and feels great joy.