WEBVTT 00:00:06.171 --> 00:00:09.930 Our story is about a girl named Iris. 00:00:09.930 --> 00:00:13.360 Iris is very sensitive. 00:00:13.360 --> 00:00:16.998 So much that she is always in tears. 00:00:16.998 --> 00:00:20.002 She cries when she's sad, 00:00:20.002 --> 00:00:23.085 when she's happy, 00:00:23.085 --> 00:00:28.020 and even tears up when things just get to her. 00:00:28.020 --> 00:00:31.363 She has special lacrimal glands to make new tears 00:00:31.363 --> 00:00:33.786 and special tubes, called lacrimal puncta, 00:00:33.786 --> 00:00:36.911 to drain old ones away. 00:00:36.911 --> 00:00:38.949 And she cries so much that she goes through 00:00:38.949 --> 00:00:41.257 ten ounces of tears per day, 00:00:41.257 --> 00:00:43.469 thirty gallons a year! 00:00:43.469 --> 00:00:45.368 In fact, if you look closely, 00:00:45.368 --> 00:00:49.971 you'll see that she's crying a little bit all the time. 00:00:49.971 --> 00:00:52.592 The basal tears that Iris constantly produces 00:00:52.592 --> 00:00:54.999 form a thin coating of three layers 00:00:54.999 --> 00:00:58.383 that cover her and keep dirt and debris away. 00:00:58.383 --> 00:01:00.915 Right next to Iris is the mucus layer, 00:01:00.915 --> 00:01:03.745 which keeps the whole thing fastened to her. 00:01:03.745 --> 00:01:06.055 On top of it is the aqueous layer, 00:01:06.055 --> 00:01:07.515 which keeps Iris hydrated, 00:01:07.515 --> 00:01:09.449 repels invasive bacteria, 00:01:09.449 --> 00:01:13.172 and protects her skin, or cornea, from damage. 00:01:13.172 --> 00:01:15.585 And, finally, there is the lipid layer, 00:01:15.585 --> 00:01:17.529 an oily outer film that keeps the surface smooth 00:01:17.529 --> 00:01:20.340 for Iris to see through, 00:01:20.340 --> 00:01:22.996 and prevents the other layers from evaporating. 00:01:22.996 --> 00:01:24.929 Normally, Iris goes about her day 00:01:24.929 --> 00:01:26.888 without really noticing the basal tears 00:01:26.888 --> 00:01:28.209 doing their thing. 00:01:28.209 --> 00:01:30.041 That's kind of their whole point. 00:01:30.041 --> 00:01:32.919 But one day, she meets a girl named Onion. 00:01:32.919 --> 00:01:34.911 Iris is immediately smitten. 00:01:34.911 --> 00:01:37.577 Onion looks gorgeous in her bright purple jacket, 00:01:37.577 --> 00:01:39.830 and she smells terrific. 00:01:39.830 --> 00:01:42.339 So, Iris invites Onion to her house for dinner, 00:01:42.339 --> 00:01:45.034 but when she comes in and takes off her jacket, 00:01:45.034 --> 00:01:47.250 something terrible happens. 00:01:47.250 --> 00:01:49.664 You see, when Onion's jacket is removed, 00:01:49.664 --> 00:01:51.499 a chemical reaction happens, 00:01:51.499 --> 00:01:54.782 converting the sulfoxides that make her smell so great 00:01:54.782 --> 00:01:56.616 into sulfenic acid, 00:01:56.616 --> 00:02:00.579 which then becomes a nasty substance with a long name: 00:02:00.579 --> 00:02:03.825 syn-Propanethial S-oxide. 00:02:03.825 --> 00:02:06.203 The gas stings Iris, 00:02:06.203 --> 00:02:07.705 and suddenenly, she can't help it, 00:02:07.705 --> 00:02:10.327 she starts weeping uncontrollably. 00:02:10.327 --> 00:02:13.164 These reflex tears are different from the basal tears 00:02:13.164 --> 00:02:14.875 that Iris is used to. 00:02:14.875 --> 00:02:16.290 Because they're designed 00:02:16.290 --> 00:02:17.833 to wash away harmful substances, or particles, 00:02:17.833 --> 00:02:21.218 they're released in much larger amounts, 00:02:21.218 --> 00:02:23.910 and their aqueous layer contains more antibodies 00:02:23.910 --> 00:02:28.249 to stop any microorganisms that may be trying to get in, as well. 00:02:28.249 --> 00:02:30.912 Both Iris and Onion are devastated. 00:02:30.912 --> 00:02:32.839 They know they can't continue their relationship 00:02:32.839 --> 00:02:34.782 if Iris is going to hurt and cry 00:02:34.782 --> 00:02:37.613 every time Onion takes off her jacket. 00:02:37.613 --> 00:02:39.522 So, they decide to break up. 00:02:39.522 --> 00:02:41.666 As Onion walks out the door, 00:02:41.666 --> 00:02:43.346 Iris stops crying. 00:02:43.346 --> 00:02:44.870 And immediately starts again. 00:02:44.870 --> 00:02:47.161 Only now, she's not crying reflex tears 00:02:47.161 --> 00:02:49.660 but emotional tears. 00:02:49.660 --> 00:02:52.591 When someone is either too sad or too happy, 00:02:52.591 --> 00:02:54.369 it feels like a loss of control, 00:02:54.369 --> 00:02:55.635 which can be dangerous. 00:02:55.635 --> 00:02:57.163 So, emotional tears are sent in 00:02:57.163 --> 00:02:58.841 to stabilize the mood 00:02:58.841 --> 00:03:00.832 as quickly as possible, 00:03:00.832 --> 00:03:02.757 along with other physical reactions, 00:03:02.757 --> 00:03:04.329 such as an increased heart rate 00:03:04.329 --> 00:03:05.921 and slower breathing. 00:03:05.921 --> 00:03:07.849 But scientists still aren't sure 00:03:07.849 --> 00:03:11.581 exactly how or why the tears themselves are helpful. 00:03:11.581 --> 00:03:13.183 They may be a social mechanism 00:03:13.183 --> 00:03:16.005 to elicit sympathy or show submission. 00:03:16.005 --> 00:03:17.818 But some studies have also found 00:03:17.818 --> 00:03:19.766 that emotional tears contain higher levels 00:03:19.766 --> 00:03:21.400 of stress hormones, 00:03:21.400 --> 00:03:24.319 such as ACTH and enkephalin, 00:03:24.319 --> 00:03:26.666 an endorphin and natural pain killer. 00:03:26.666 --> 00:03:30.423 In this case, emotional tears are also directly calming Iris down, 00:03:30.423 --> 00:03:33.416 as well as signaling her emotional state to others. 00:03:33.416 --> 00:03:35.703 Sorry things didn't work out with Onion, Iris, 00:03:35.703 --> 00:03:37.085 but don't worry. 00:03:37.085 --> 00:03:39.411 As long as you have all three kinds of tears 00:03:39.411 --> 00:03:41.172 working to keep you balanced and healthy, 00:03:41.172 --> 00:03:42.300 it will get better. 00:03:42.300 --> 00:03:43.017 You'll see.