The first five minutes of a sub day decide how the rest of the period goes. If students walk in and the routine looks normal, most of them settle in. If the board is blank and the substitute is hunting for papers, you'll hear about it when you get back. A bell ringer is the simplest way to protect that opening.
Use the same format students already know
A sub day is not the time to try a new warm-up. Use the exact format your class does every day: same spot on the board, same notebook page, same timer. Students who know the routine will start without being told, and that helps the substitute more than any note you leave.
Write it so anyone can run it
Your sub may not know your subject. Write the bell ringer so it needs no explanation:
The prompt, written out in full
Where students write their answer
How long they have
What to do when time is up (collect, or keep in notebooks)
Skip anything that requires the sub to teach content or judge answers on the spot.
Choose prompts that don't need you
Reflection and review prompts work best on sub days. "Write three things you remember from yesterday's lesson." "Which question from this week's quiz would you still get wrong, and why?" Students can answer these without new instruction, and you get useful information when you're back.
Collect the answers
Ask the substitute to collect bell ringers at the end of class. It tells students the work counts, and it gives you a quick look at who was working. Pair it with an end-of-class check, like a 3-2-1 exit ticket, and you'll know exactly where to pick up.
Keep a ready-made backup
Emergencies don't give you time to write a plan. Keep a folder of bell ringers and a full day's activities ready to go. SubstitutePlans.com has emergency sub plans, a free emergency sub plans PDF, and plans by subject. Daily Bell Ringers posts new bell ringers every school day that a sub can pull up and project.
Keep going
Need something ready to use tomorrow? The Teacher Emergency Toolkit has grab-and-go packs for de-escalation, work refusal, behavior documentation, and sub plans.