When a volcano “erupts”, it sprays up into the air a stream of red-hot melted rock, called lava. Well, nature does the same thing with water. A “geyser” is a burst of underground water that shoots up through the ground. One of the most famous geysers is Old Faithful in Yellowstone National Park. We call it Old Faithful because it’s always there for us: it erupts every 91 minutes, shooting boiling hot water and steam 185 feet into the air for up to 5 minutes at a time! In the old days, people tried to use Old Faithful to wash their clothes, but the blasting water tore everything to shreds. Because of changes underground, the length of time each burst lasts has changed over the years. But Old Faithful still deserves its name, as it still explodes about every hour and a half — whether you’re waiting to do laundry or not.
Wee ones: If the boiling water has been spraying for 4 minutes and has 1 more minute to go, for how long will it spray in total?
Little kids: If Old Faithful is about to start spraying in 10 seconds, what numbers do you say for the countdown, starting with 10? Bonus: The water sprays 185 feet into the air. Is that closer in height to a person or a 12-story building?
Big kids: If an eruption ends at 3:10 pm, at what time will you see the next one if it starts 91 minutes after that? (Reminder: there are 60 minutes in an hour.) Bonus: If people try to wash their laundry at Old Faithful every 3rd day starting on a Monday, how many days later will they get to wash laundry on a Tuesday?
Wee ones: 5 minutes.
Little kids: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1. Bonus: The 12-story building.
Big kids: 4:41 pm, since the first 60 minutes will bring you to 4:10 pm. Bonus: 15 days later. You need a Tuesday that is a multiple of 3 from Monday; the next day is 1 day, the following Tuesday is 8 days, and the Tuesday after that is finally 15 days (2 weeks 1 day).