*,
*::before,
*::after {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  background: #fff;
  color: #111;
  font-family: "Segoe UI", system-ui, sans-serif;
  font-size: 16px;
  line-height: 1.4;
}

.scene {
  position: relative;
  display: flex;
  align-items: flex-start;
  overflow: visible;
}

.scene-leader,
.pressure-leader {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  overflow: visible;
  pointer-events: none;
  z-index: 3;
  color: #111;
}

.pressure-leader-line,
.force-pressure-leader-line,
.bottom-pressure-leader-line,
.buoyant-leader-line,
.floater-weight-leader-line {
  fill: none;
  stroke: currentColor;
  stroke-width: 1.6;
  stroke-linecap: round;
}

.tank {
  width: 300px;
  height: 590px;
  background: #e8f1fc;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  flex-shrink: 0;
  overflow: visible;
}

.cylinder {
  width: 104px;
  height: 420px;
  margin-top: 10px;
  overflow: visible;
}

.copy {
  display: grid;
  grid-template-columns: auto 9.6rem auto;
  column-gap: 0.85rem;
  row-gap: calc(2.2rem - 5px);
  margin-top: 2.7rem;
  margin-left: -7.4rem;
  align-items: center;
}

.arrow {
  grid-column: 1;
  grid-row: 1;
  position: relative;
  width: 11.5rem;
  height: 1.5px;
  background: #111;
}

.arrow::before {
  content: "";
  position: absolute;
  left: 0;
  top: 50%;
  transform: translate(-1px, -50%);
  border-top: 5px solid transparent;
  border-bottom: 5px solid transparent;
  border-right: 8px solid #111;
}

.force-label {
  grid-column: 2;
  grid-row: 1;
  margin: 0;
  white-space: nowrap;
}

.first-eq {
  grid-column: 3;
  grid-row: 1;
  position: relative;
  min-height: 1.4em;
}

.first-eq p,
.eq {
  margin: 0;
  white-space: nowrap;
}

.eq[data-show-at="6"] {
  grid-column: 3;
  grid-row: 2;
}

.eq[data-show-at="8"],
.expand-eq {
  grid-column: 3;
  grid-row: 3;
}

.volume-eq {
  position: relative;
  padding-top: 2.6rem;
}

.volume-term {
  position: relative;
  display: inline-block;
}

.term-swap {
  position: relative;
  display: inline-block;
  vertical-align: baseline;
}

.term-swap .term-to {
  position: absolute;
  left: 0;
  top: calc(100% + 0.1em);
}

.term-swap.is-swapped .term-from {
  display: none;
}

.term-swap.is-swapped .term-to {
  position: static;
}

.callout {
  position: absolute;
  margin: 0;
  padding: 0.35rem 0.7rem;
  border: 1px solid #222;
  background: #fff;
  white-space: nowrap;
  font-size: 0.95rem;
}

.callout::after {
  content: "";
  position: absolute;
  top: 100%;
  border: 6px solid transparent;
  border-top-color: #222;
}

.mass-callout {
  left: 1.15em;
  bottom: calc(100% + 14px);
}

.mass-callout::after {
  left: 1.35rem;
}

.volume-callout {
  left: 50%;
  bottom: calc(100% + 14px);
  transform: translateX(-50%);
}

.volume-callout::after {
  left: 50%;
  transform: translateX(-50%);
}

.pressure-label {
  grid-column: 2;
  grid-row: 2;
  align-self: start;
  margin: 0;
}

.pressure-eq {
  grid-column: 3;
  grid-row: 2;
  align-self: start;
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 1.35rem;
  margin-top: 0;
}

.pressure-eq-sub {
  display: flex;
}

.sub-col {
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 1.35rem;
}

.sub-row {
  display: flex;
  align-items: center;
  gap: 0.65rem;
}

.simplified-eq {
  margin: 0;
}

.force-pressure-label {
  grid-column: 2;
  grid-row: 3;
  align-self: start;
  margin: 0;
}

.force-pressure-eq {
  grid-column: 3;
  grid-row: 3;
  align-self: start;
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 1.35rem;
  margin-top: 0;
}

.bottom-pressure-label {
  grid-column: 2;
  grid-row: 4;
  align-self: start;
  margin: 0;
}

.buoyant-label {
  grid-column: 2;
  grid-row: 5;
  align-self: start;
  margin: 0;
  white-space: nowrap;
}

.buoyant-eq {
  grid-column: 3;
  grid-row: 5;
  align-self: start;
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 1.35rem;
  margin-top: 0;
}

.floater-weight-label {
  grid-column: 2;
  grid-row: 6;
  align-self: start;
  margin: 0;
}

.floater-weight-eq {
  grid-column: 3;
  grid-row: 6;
  align-self: start;
  margin-top: 0;
}

.neutral-label {
  grid-column: 2;
  grid-row: 7;
  align-self: start;
  margin: 0;
  white-space: nowrap;
}

.neutral-eq {
  grid-column: 3;
  grid-row: 7;
  align-self: start;
  margin-top: 0;
}

.neutral-sub {
  grid-column: 3;
  grid-row: 8;
  align-self: start;
  margin-top: 1.35rem;
}

.bottom-pressure-eq {
  grid-column: 3;
  grid-row: 4;
  align-self: start;
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 1.35rem;
  margin-top: 0;
}

.fraction-wrap {
  position: relative;
  display: inline-flex;
  overflow: visible;
}

.cross-out {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  overflow: visible;
  pointer-events: none;
  z-index: 2;
}

.cross-out line {
  stroke: #c43c3c;
  stroke-width: 2;
  stroke-linecap: round;
  fill: none;
  stroke-dasharray: 1;
  stroke-dashoffset: 1;
}

.cross-out.is-visible line {
  animation: draw-cross 0.45s ease forwards;
}

@keyframes draw-cross {
  from {
    stroke-dashoffset: 1;
  }

  to {
    stroke-dashoffset: 0;
  }
}

.fraction {
  display: flex;
  flex-direction: column;
  align-items: center;
  text-align: center;
}

.fraction .num {
  padding: 0 0.35rem 0.15rem;
}

.fraction .bar {
  display: block;
  align-self: stretch;
  height: 1.5px;
  background: #111;
}

.fraction .den {
  padding: 0.2rem 0.35rem 0;
}

.weight-arrow {
  fill: #fff;
  stroke: #1a1a1a;
}

.buoyant-arrow {
  fill: #fff;
  stroke: #1a1a1a;
  stroke-width: 1.3;
  stroke-linejoin: round;
}

.buoyant-arrow.is-pulsing {
  animation: buoyant-arrow-pulse 1.4s ease-in-out infinite;
}

@keyframes buoyant-arrow-pulse {
  0%,
  100% {
    fill: #8fd4a4;
    stroke: #2d7a4a;
  }

  50% {
    fill: #1f6b3a;
    stroke: #0d3d20;
  }
}

.floater-weight-arrow {
  fill: #fff;
  stroke: #1a1a1a;
}

.floater-weight-arrow.is-pulsing {
  animation: floater-weight-arrow-pulse 1.4s ease-in-out infinite;
}

@keyframes floater-weight-arrow-pulse {
  0%,
  100% {
    fill: #d4a8e8;
    stroke: #7a3d9c;
  }

  50% {
    fill: #6b1f9a;
    stroke: #3d0d5c;
  }
}

.weight-arrow.is-pulsing {
  animation: weight-arrow-pulse 1.4s ease-in-out infinite;
}

@keyframes weight-arrow-pulse {
  0%,
  100% {
    fill: #8fb0d8;
    stroke: #2c4f7c;
  }

  50% {
    fill: #1a3f73;
    stroke: #0c2448;
  }
}

.pressure-arrows,
.bottom-pressure-arrows {
  fill: #f2d36a;
  stroke: #c4a33a;
  stroke-width: 0.35;
  stroke-linejoin: round;
}

.bottom-surface {
  fill: rgba(196, 60, 60, 0.2);
  stroke: #c43c3c;
  stroke-width: 1.4;
  stroke-dasharray: 3.5 2.5;
}

.water-surface {
  transform-box: fill-box;
  transform-origin: center;
}

.water-surface.is-pulsing {
  animation: pressure-pulse 1.4s ease-in-out infinite;
}

@keyframes pressure-pulse {
  0%,
  100% {
    fill: #fff;
    stroke: #1a1a1a;
    stroke-width: 1.2;
  }

  50% {
    fill: #ffd4d4;
    stroke: #c43c3c;
    stroke-width: 2.6;
  }
}

.ghost-target {
  white-space: nowrap;
}

.ghost-target.is-waiting {
  opacity: 0;
  visibility: hidden;
}

.ghost {
  position: fixed;
  margin: 0;
  white-space: nowrap;
  pointer-events: none;
  z-index: 20;
  color: #111;
  opacity: 0.45;
  transition:
    left 0.85s cubic-bezier(0.4, 0, 0.2, 1),
    top 0.85s cubic-bezier(0.4, 0, 0.2, 1),
    opacity 0.28s ease;
}

[data-show-at]:not(.is-visible),
[data-hide-at].is-hidden,
.mass-gravity-term.is-hidden,
.pressure-area-term.is-hidden,
.pressure-area-term-bottom.is-hidden,
.buoyant-diff-term.is-hidden {
  display: none;
}

.controls {
  margin-top: 3.5rem;
  display: flex;
  justify-content: center;
  gap: 0.75rem;
}

.controls button {
  font: inherit;
  font-size: 0.95rem;
  font-weight: 500;
  padding: 0.65rem 1.45rem;
  border: none;
  border-radius: 999px;
  cursor: pointer;
  transition: background 0.15s ease, color 0.15s ease, opacity 0.15s ease;
}

#back {
  background: #eef1f4;
  color: #222;
}

#back:hover:not(:disabled) {
  background: #e2e6ea;
}

#next {
  background: #171717;
  color: #fff;
}

#next:hover:not(:disabled) {
  background: #333;
}

.controls button:disabled {
  cursor: default;
  opacity: 0.4;
}