The order you do things in decides what fits.

Same pile. Different order. Watch what fits.

Both cups receive an identical set of pieces — each piece keeps its size and its importance in both runs. The only difference is the order they're dropped in. Pieces that settle above the brim line didn't make it in. Hit play and watch the gap open up.

A genuine little physics simulation — same pieces, same drop positions, same gravity in both cups. The brim line is each cup's fixed capacity; the only variable is the order. Watch the twist: the random cup often ends up busier (more pieces in it) yet keeps less of what matters, because it spends the scarce space on whatever arrived first and lets big rocks spill over the brim. The big-rocks cup keeps the rocks and still lets sand trickle into the crevices.

Two takeaways the jar keeps proving.

This demonstration is usually done with literal rocks, gravel and sand in a glass jar. The lesson survives the move to physics because it was never really about jars. Tap a card for the longer version.

The same idea, proven in computer science.

"Big rocks first" isn't just a motivational metaphor — it's a known, provably-good strategy for a whole family of packing and scheduling problems. The cup is a container; the rocks are weighted items; "what fits" is an optimisation. Here's where the maths backs the metaphor.

Where this comes from.

The story has two lineages — the management one and the algorithmic one. Both say the same thing: decide what matters, do it first, and let the rest fill in around it.

• Stephen R. Covey, A. Roger Merrill & Rebecca R. Merrill — First Things First (1994). The origin of the "big rocks" jar demonstration: rocks first, then gravel, pebbles, sand and water fill the gaps. The point isn't "you can cram more in" — it's that "if I hadn't put those big rocks in first, I'd never have got them in at all." Covey credits the unnamed teacher who showed him; the story's exact origin is unknown. Read the passage →
• Peter F. Drucker — The Effective Executive (1966). The deeper root. Drucker separates effectiveness (doing the right things) from efficiency (doing things right), and devotes a chapter to "First Things First": effective executives "do first things first and second things not at all." Concentration on the few things that matter is, for Drucker, the secret of getting results. About the book →
• Bin packing & First-Fit-Decreasing. The algorithmic twin of "big rocks first." Sorting items largest-first before packing is provably near-optimal — the tight bound is FFD(I) ≤ (11/9)·OPT(I) + 6/9 (Dósa, 2007). Arbitrary order has no such guarantee. Bin packing →
• The fractional knapsack problem. When you must choose what to carry in limited space, the greedy rule "take the highest value-per-size first" is mathematically optimal. Importance density beats raw size or raw value alone. Fractional knapsack →

The hard part was never the jar. It's deciding, honestly, which of your rocks are actually rocks — and protecting the order against a week that wants to hand you sand first. That's the conversation our strategy sessions are built around.

Deciding what fits is only half of it.

Order protects the big rocks. The next question is whether the rocks are worth carrying at all — which is where these two go next.