I can give you several sentences.
I/you/we/they stiffen. He/she/it stiffens.
Loosen up is the opposite of stiffen up.
The rubber gaskets are usually pliant but stiffen in very cold temperatures. (metaphor) The pliant minds of the children accepted the sea captain's tall tales as true stories.
Use a heavier weight
lack of potassium
they clean the insides of the body then they use this pacific kind of liquid to stiffen the body and rap cloth around the body.
set, solidify, stiffen, :)
stiff
the molecues stiffen up
Yes it normal for infants to stiffen when crying or screaming. It is caused by the action of crying (try screaming really loud and you will find that you tense your body), they also stiffen if they do not want to be picked up. As long as the stiffness does not last beyond the crying there is nothing to worry about.
i do not know how to use embalming in a sentence. (there is the sentence)
So- you are asking when to use 'when' in a sentence. When you are asking how to use when in a question, you are already using when in a sentence, because a question actually is a sentence. I like to use when in a sentence whenever I like.