Poop knife was a meme about something else, but to answer you, this is how you can do it.

0. Flush.
1. Get a strip of toilet paper.
2. Lay it such that it is over the poo on the edge and also in the water.
3. Because you recently flushed and the end is in water, the entire strip of toilet paper should now be wet and clinging to the edge.
4. Flush again. The toilet paper gets sucked and/or pushed down and should get the poo with it.