"I have searched the science stockroom from top to bottom and there is absolutely no trace of it," I tell the science coordinator.
"No trace of what?" she asks.
"Sweltered venom of a cold toad," I reply.
If she was any more nonplussed, she would be in danger of becoming a minus.
"Neither can I find any eye of newt, toe of frog, wool of bat or tongue of dog. And while we're on the subject, we appear to be completely out of fenny snake fillets. What kind of science leader are you?" I ask.
Luckily, Grendel is not the sort who quickly turns to violence, and understands when I explain that I want to do a practical science lesson with cross-curricular links to literacy and drama.
"We are writing our own 'Double, double toil and trouble' poems for Halloween," I explain. "And we are studying irreversible changes in science. I want to link them together by making a witch's brew; something that - at the risk of contravening safeguarding regulations - boils and bubbles to dramatic effect."
Grendel suggests foaming cauldrons, a variation on the old erupting volcano activity.
First, the children make their cauldrons using black plasticine (clay, salt dough or papier mache will also do). Then, using their thumbs, they form a small cavity in the top, just deep enough to throw in a few hellish ingredients.
Only when their cauldrons and poems are perfected do we get down to the fun stuff. In between shrill chants of "Double, double toil and trouble", First Witch throws in the ground bones of a raven, Second Witch adds extract of squashed frog, and Third Witch drops in vile venom of a vicious viper. Now all three hags cackle outrageously as their brew spews its evil contents over the table and fills the air with foul vapours.
During our science of witchcraft debriefing session, the secret is revealed. Our magical ingredients turn out to be nothing more sinister than baking soda (sodium bicarbonate), vinegar (acetic acid) and green food dye for added dramatic effect.
The children are shown how the acetic acid in vinegar reacts with baking soda to produce carbon dioxide, the gas responsible for all the fizzing and bubbling. They then learn that this is an irreversible change because the reaction leads to new materials being formed.
"Unfortunately, now you know my secrets I will have to kill you all," I explain to the children when the lesson is over.
"Not if I turn you into a frog first," shrieks Grendel, who has entered dressed for Halloween. "But you're a scientist," I cry. "You know it's impossible to turn a teacher into a... ribbit... "
Steve Eddison is a key stage 2 teacher at Arbourthorne Community Primary School in Sheffield
Try erhgiez's 11-lesson scheme of work, workbook and PowerPoint to help you teach reversible and irreversible changes.
Then test pupils' knowledge using homework sheets shared by mwiggins. bit.lyIrreversibleChanges.