Turn to jelly

From: http://www.bbc.co.uk/worldservice/learningenglish/language/2014/05/140513_todays_phrase_turn_to_jelly.shtml

Life-size jelly babies

Imagine trying to eat a jelly baby this big!

Today's Phrase

If you turn to jelly, it really means you suddenly feel very weak because you are nervous or frightened. Sometimes you can say your legs or another part of your body turns to jelly and this means the same thing.
Examples:
When I saw how high up we were, I turned to jelly and I couldn't speak.
His legs turned to jelly after riding the ghost train at the funfair!

Take note

If something is like nailing jelly to the wall, it is impossible to understand or describe exactly what it is.

Example:
Writing the history of this period is like nailing jelly to the wall.

Interesting fact

In the UK, jelly describes a soft sweet food made from fruit juice, sugar and gelatine which you can see through and that shakes when you touch it. It's often eaten at children's parties. But in the USA jelly describes a sweet, sticky substance made with boiled fruit and sugar and is spread on bread. The British would call this 'jam'.