1. “Do bath bomb expire?”

Bath bomb will get less effective as time goes on. The fresher they are, the faster they’ll fizz, says Jack Constantine, head of product development at Lush.

“We have fanatics who’ll keep theirs wrapped and in tupperware boxes underneath their bed, for years – which is extreme,” he says. “But you can do it, as it’s about the freshness. If the bath bomb are out on display, they’re oxidising and they’re losing the ability to react so well. So the citric and the bicarb start to bind. The fresher they are, the faster they fizz.

“Our freshness policy in the factory is 21 days. So if you buy off the website, it’s normally seven days at a push [your product has been made within seven days]. We will get complaints from people who say, ‘I bought this a year ago, I chucked it in and it did nothing!’ You need to use it. The same with the smell – the smell will wear off as all the essential oils start to evaporate.”

“Where you have one mix in the centre that’s bursting to get out, once the mix finds a hole, that starts propelling it,” he says. “But it finds a hole in different places, so every time you chuck it in, it’ll be different. You’ll never get the same thing.

“The best thing with those kind of bath bombs is to not get in. Fill your bath, chuck it in, and wait five minutes. And then watch it, then get in.”

3. “How do I make the colors of my bath bomb swirl?”

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The chemical reaction between bicarbonate of soda and citric is what makes your bath bomb fizz and move across the tub, according to Lush digital lab assistant Rachel Lake. One mixture is faster than the other, so by combining the mixture in certain ways, the inventors can control the flow of the bath bomb.

“The colors start to swirl and move around because of the chemical reaction between the bicarb and the citric,” she says. “When that fizzes in the water, the bomb can turn around – say if you put a bubble mix on one side, and a ballistic mix on the other, that’s going to push it.”

Jack adds: “For example, we can force the mixes to go in different directions. We can control the flow of the bath bomb. There are patents on these two separate mixes that we use, and when we combine them one’s faster than the other.”

4. “How do I get glitter off my bathtub?”

“Wash it!” Jack says. “Bath bomb, especially the ones I work on, have a mix that has some surfactant in it. That helps to remove it from the edges, naturally. But if it’s getting stuck on there, we always suggest a shampoo bar, or a shower gel, whatever.”

Surfactant are compounds used as foaming agents, detergents, and emulsifiers. “You don’t need serious toilet products to clean it,” he says. “Anything with a little bit of surfactant in it will just wash it off. The same with the colour – often we’ll get concerns like, ‘Oh, you’ve stained my bath.’ But if you just use a product with surfactant in it, it’ll come straight off.”