Possible Clues:

• Is the one that adds about to fall?
• Seeing that an adder has no legs it can never even go like this to reach a 24 across
• Having no feet, the adder can't even do this
• Stagger, reel
• The sin of being grown up?
• Having no feet, the adder cannot even do this
• After a little 23 down, the beast may do this before the fall
• Waver and threaten to fall
• Walk unsteadily as if about to fall
• If one is about to fall for this, one should be able to find what it all adds up to