The kids could use a variety of playdough colours, or just one colour and some crafty bits and pieces, to model a big letter X. It's a great way to get them familiar with the shape of the letter.
Alphabet Decorate the Letter X Playdough Mat Upper case
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Alphabet Playdough Mat X
Why not make working with playdough one of your "letter of the week" activities - and print out this playdough mat, for the letter X, to help!
Alphabet Begin With The Letter X Playdough Mat
I'm not sure how you model an X-Ray out of playdough - but perhaps the kids can come up with a way? You might relax the rules with this playdough mat and ask them to model words ending with the letter x, too.
Alphabet Decorate The Letter X Playdough Mat Lower Case
How will the kids make the letter X out of playdough? Some might make two "sausages" and cross them over - as if they were writing the letter.