On not using mnemonics, and don’t get me started on crocodiles.

I’ve just seen another example of a mnemonic for remembering domain knowledge that reveals a potential lack of understanding of the subject matter, or a shortcoming in teacher pedagogy…

“To remember which binary shift does what, use this mnemonic: Lemon Meringue is Really Delicious – Left Multiply, Right Divide” (I have changed the mnemonic to protect the original poster).

I worry about any class learning this mnemonic as it suggests they are not being taught what binary shifts do, or why shifting multiplies or divides, and how this behaviour is exactly the same in any number base.

I teach binary by starting with place value and explaining how the places each have a value based on their position, which goes up by 2 each time as we move left, because we are in base 2, and show how this is exactly the same as in denary just with a different multiplier (10). When doing shifts I then do lots of worked examples in both denary and binary, and convert each binary result back to denary to illustrate the multiplying by 2 and dividing by 2 that is happening.


In this way, the binary number system becomes demystified – it’s just the same as base 10 just with different place values and a reduced symbol set (0 and 1). So they totally get that moving digits right (to the less significant places, the smaller column headings) makes a number smaller, and they understand that moving digits left (towards the more significant places, the larger column headings) makes it bigger.

Using a mnemonic suggests that the core concept of place value has not been taught or learned.

I feel the same about teaching greater than / less than by using “the crocodile eats the other number“, and about mnemonics like “rOm is nOn-volatile”. The < and > symbols are representations of growing and shrinking, the symbol is larger on one side than the other. Why add complexity with some notional crocodile? And ROM means “read-only memory” so it cannot be written to, so if it was volatile (lost its contents on power down) then it would be a pretty useless wafer of silicon as it would remain empty forever more. If they understand what ROM really is, and what volatile means* they cannot get this one wrong.

* I teach “volatile” with reference to Chemistry, where it means “easily evaporated”. If something evaporates it is lost, like the contents of volatile memory.


Relying on tricks can sometimes add complexity and confuse: One maths teacher told me that one of their students “got the greater than and less than symbols backward, because she believed the larger number would therefore be in the crocodile’s stomach after he ate it”.

Just teach the subject knowledge. Teach it well. And check for understanding. No crocodiles please.


By mraharrisoncs

Freelance consultant, teacher and author, professional development lead for the NCCE, CAS Master Teacher, Computer Science lecturer.

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