"But its different with the numbers!"

One of my students who I would consider one of my top students, is defending her opinion that she can subtract 24 from 18 and get a positive number.   She's telling the class you just have to carry!

I write the problem vertically on the board and ask her to come explain what she means.   I want her to figure it out or have another student pinpoint her mistake, but it's not happening.  

I draw base ten blocks and start illustrating what she says.  She is convinced and getting more convinced each time that she is correct.  None of the other students are speaking up. 

Finally I take out 18 cubes and ask her to take away 24 of them.  She confidently removes the blocks one by one and counts.  When she gets to 18 and there are no more she looks at me and declares "But its different with the numbers!!" 

Wow!