prall
plump / taut
Filled to the maximum and therefore firm and tightly stretched. Often used for fruit, balls, tires, or body parts.
Der Fahrradreifen ist prall mit Luft gefüllt.
The bicycle tire is filled full of air and taut.

plump / blazing
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plump / taut
Filled to the maximum and therefore firm and tightly stretched. Often used for fruit, balls, tires, or body parts.
Der Fahrradreifen ist prall mit Luft gefüllt.
The bicycle tire is filled full of air and taut.

'Prall' (plump/taut) is often combined with things that have a round, full shape. Typical examples are plump fruit (cherries, tomatoes), a taut ball, or a wallet stuffed full.
Die Kirschen am Baum sind dieses Jahr besonders rot und prall.
The cherries on the tree are especially red and plump this year.
The combination 'in der prallen Sonne' (in the blazing sun) is a fixed expression. Here, 'prall' does not mean 'filled', but rather 'direct', 'intense', and 'without shade'.
Man sollte mittags nicht in der prallen Sonne Sport machen.
You shouldn't exercise at midday in the blazing sun.
The expression 'das pralle Leben' (life in all its fullness) is figurative and describes a life full of intensity, energy, and events. It is often used to characterize dynamic life in a city or in a lively place.
Wer das pralle Leben sucht, zieht nach Berlin.
Anyone looking for life in all its fullness moves to Berlin.
direct
'direkt' and 'prall' can both refer to sun or light, but they are used differently. 'direkt' generally means that something hits something without a detour or without protection. For example, you can say that the sun shines directly into the room. In this context, 'prall' is used mainly in the fixed phrase 'in der prallen Sonne' (in the blazing sun). This emphasizes strong, intense, and unprotected sunlight. The words can be confusing because 'pralle Sonne' basically also means direct sun. Still, 'prall' is much narrower and does not work generally for every direct effect.

Im Sommer sollte man nicht zu lange in der prallen Sonne sitzen.
In summer, you should not sit in the blazing sun for too long.
Am Nachmittag scheint die Sonne direkt auf meinen Schreibtisch.
In the afternoon, the sun shines directly onto my desk.