上一片，小电脑学会了转圈干活。小卖部又开了果汁台：每杯都要洗、切、榨、倒。同样四步，写了一遍又一遍。这一片，把它们打包成一台机器。Last leaf, the little computer learned to run laps. Now the snack stand adds a juice bar: wash, cut, squeeze, pour — for every cup. The same four steps, written again and again. This leaf packs them into one machine.
第 1 步：一台榨汁机Step 1: One juicer
同样的四步，能只写一次吗？Can the same four steps be written just once?
能！函数就是打包好的一段步骤。像榨汁机：水果进，果汁出。里面怎么洗、怎么切，不用再操心。给它起个名字，比如 榨汁()。喊一声名字就开工，这叫“调用”。Yes! A function is a packed-up set of steps. Like a juicer: fruit in, juice out. How it washes and cuts is no longer your worry. Give it a name, like juice(). Say the name and it starts — that is a “call”.
每次的水果不一样，机器怎么知道？Different fruit each time — how does the machine know?
第 2 步：参数进，返回值出Step 2: Parameter in, return value out
靠参数！参数是你递进去的东西。返回值是机器交回来的结果。图里这台会翻倍：放进 4，交回 8。放进 5，就交回 10。同一台机器，参数一换，结果就变。Parameters! A parameter is what you hand in. The return value is what the machine hands back. This one doubles: put in 4, get back 8. Put in 5, get back 10. Same machine — change the parameter, change the result.
打包好的机器，还能搬去哪儿用？Where else can a packed machine go?
第 3 步：写一次，到处用Step 3: Write once, use everywhere
哪儿都能用！店里、家里、学校，用的是同一张配方。要改配方，只改一处，处处跟着变。程序员天天用函数：print 打印，max 找最大。大程序，就是很多小函数拼起来的。Anywhere! Shop, home, school — all share one recipe card. Change the recipe once, and every copy follows. Coders use functions all day: print shows text, max finds the biggest. Big programs are many small functions snapped together.
🎮 你来开机器（30 秒）🎮 Your turn: run the machine (30 seconds)
道理讲完了。先选水果、拨杯数，调用几次榨汁机。再接 4 关：猜猜机器交回什么。Theory done. First pick a fruit and the cups, and call the juicer a few times; then take 4 rounds: guess what the machine hands back.
一句话记住它：函数 = 打包好、有名字的一段步骤；参数递进去，返回值交回来，写一次到处用。Remember it in one line: a function = a named, packed-up set of steps; parameters go in, a return value comes out — write once, use everywhere.
喊名字就开工：调用 榨汁()Say its name to start it: call juice()参数进，返回值出：翻倍(4) → 8Parameter in, return value out: double(4) → 8写一次到处用：改一处，处处跟着变Write once, use everywhere: fix once, fixed everywhere
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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