## 4 Conclusions  
We have presented a practical way of converting pencil waste into functional carbon materials with the aid of microwaves. The strong interaction of graphite lead with microwaves produces hot plasma, causing the carbonization of the pencil's wood casting. Without chemical activation, an important class of carbon nanomaterials is obtained, namely carbon nanosheets. However, chemical activation with KOH leads to porous carbon particles that can decolorize dye solutions, produce heat in pocket warmers based on iron and carbon, fuel gunpowder and efficiently remove hexavalent chromium from water under acidic conditions. In both cases (with or without activation), the derived carbons are amorphous and oxidized; i.e., they bear oxygen-containing functionalities at the surface, which may facilitate further post-functionalization of the solids towards reconstructed derivatives. The method is simple, fast (e.g., completes within 1-2 min of irradiation) and cost-effective due to the microwave technology involved, whereas the excellent absorptivity of wood and its ability to take up a variety of fluids/solutions is soaked in, and it provides additional options in the chemical activation or surface modification of the wood pencils derived carbons. By considering the number of pencils used per year (14 billion), their average mass when discarded (i.e., 40% of the average mass of a pencil, the latter being about 4 g) and the carbon yield (3-4%), it is roughly estimated that nearly 500-1000 metric tons of carbon could be annually produced this way, finding numerous useful applications in water purification filters, pocket warmers or gunpowder manufacture. Besides writing and drawing, pencils could be, after all, the subject of fundamental carbon science and technology without sophisticated requirements.  
Conceptualization, A.B.B.; methodology, N.C., A.B.B., M.A.K. and C.E.S.; validation, A.B.B., N.C., G.A., M.B., M.A.K. and D.G.; investigation, M.B., A.B.B., C.E.S., M.A.K. and D.G.; data curation, N.C., A.B.B., D.M., G.A., M.B., A.A., I.T., A.B., C.E.S., M.A.K. and D.G.; writing--original draft preparation, A.B.B., C.E.S., N.C. and M.A.K.; writing--review and editing, A.B.B., C.E.S. and M.A.K.; supervision, A.B.B., M.A.K. and D.G. All authors have read and agreed to the published version of the manuscript.  
Figure 13: Deconvoluted high-resolution Cr2p spectrum of the APC sample after chromium adsorption.  
**Funding:** This study was funded by the project \"National Infrastructure in Nanotechnology, Advanced Materials and Micro-/Nanoelectronics\" (MIS-5002772), which was implemented under the action, \"Reinforcement of the Research and Innovation Infrastructure\", funded by the Operational Programme \"Competitiveness, Entrepreneurship and Innovation\" (NSRF 2014-2020), and co-financed by Greece and the European Union (European Regional Development Fund). N.C. gratefully acknowledges the IKY foundation for their financial support. This research was also co-financed by Greece and the European Union (European Social Fund-ESF) through the Operational Programme \"Human Resources Development, Education and Lifelong Learning\" in the context of the project \"Strengthening Human Resources Research Potential via Doctorate Research\" (MIS-5000432), implemented by the State Scholarships Foundation (IKY). I. T. acknowledges the support by the Operational Programme Research, Development and Education-Project No. CZ.02.1.01/0.0/0.0/15_003/0000416 of the Ministry of Education, Youth and Sports of the Czech Republic. A. B. acknowledges the funding from the Czech Science Foundation, project GA CR-EXPRO, 19-27454X.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding authors.  
**Acknowledgments:** The authors greatly acknowledge [PERSON] for the XRD and [PERSON] for the XPS measurements.  
**Conflicts of Interest:** The authors declare no conflict of interest.