Why do I need to know piquer une tête? 

Because you’ll need to know what to say when it gets too hot at the pool. 

What does it mean? 

Its literal translation is 'sting' or 'poke a head', but it refers to to 'plunging head first' into water. 

Piquer une tête is used to say that you are diving into the water, literally meaning that you are going into the water with your head first. It can also simply mean throwing yourself into the water. 

Usually piquer une tête just means taking a dip. 

It can also be used when you are throwing yourself on the ground in a situation of crisis (for example during a bank robbery). But mostly it's used for the fun water-related sense.

Use it like this 

Après, on ira piquer une tête dans la mer  – Afterwards, we’ll go have a dip in the sea. 

Bon allez, je vais piquer une tête, il fait beaucoup trop chaud ici – Ok, I’m going for a swim, it’s way too hot here. 

Synonyms

Sauter à l'eau – jump into the water