I have been juggling with Thuds for years now. They are for many jugglers the staple prop in their arsenal, right beside the flaming torches and razor sharp knives.

The Balls
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Thuds are made of 4 coloured panels of vinyl covered material, containing about 120g of seed. The material is elastic so they squash nicely in your hand making them easy to catch and control. They come in a variety of sizes from tiny to huge 1kg monsters, but the standard size is a little smaller than a tennis ball. Young children may find them a little large so the smaller version may be more appropriate. They generally cost around £2-3 each depending on where you buy them.

Juggleability
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These are widely used amongst jugglers for a reason. They are a good weight for comfortably juggling anywhere from 3 to 5 balls for extended periods of time without getting worn out. It is possible (with a bit of practise) to juggle 7 of these, although I prefer to use something smaller and lighter when attempting this many. They good for passing between jugglers as well and because they're soft won't cause injury when they hit you/someone else accidentally.

Durability
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A set of thuds will last for several years if you treat them nicely. I've had a couple of sets now and whilst I've never had one burst on me the vinyl covering will eventually start to crack and peel. When this happens the ball starts to loose it shape. This can be avoided or at least delayed by not dropping the balls onto concrete. Better still never drop them at all. Having juggled near a swimming pool I'm can confirm that these balls do float, although I'm sure if you leave them in the pool long enough you'll be diving to the bottom of the deep end to get them.

I would warn everyone that if you give these to teenage boys they will, with complete certainty, end up throwing them at each other.

You really can't go too far wrong with a set of these. They remove any excuses that other inferior balls might offer, so you won't be able to blame any shortcomings in your juggling skill on the balls!