Top 10
In reverse order:

10. "Upper bounds, they are above me."
9. "All the set theory we've done is completely useless."
8. "You must do all your maths work and thinking whilst in the bath tub."
7."It's called curried because it's a bit spicy."
6. "Once I tried to seduce a girl in a maths lecture but I wasn't successful."
5. "What is a proof? I do not know what is a proof!"
4. "An old bloke. Very old. Because he's already dead."
3. "I'm going to tell you a story that's quite interesting.... there's a lot of dead people involved."
2. "Often you have to know what people are talking about to know what they mean."

AND STILL AT NUMBER ONE...

1. "What is the lowest random number? Seven.... It's a joke, no?"
 
 

Categorized Quotes
Contains the ones that didn't quite make the top 10:

Sets

• "What is x?"
• "x - a nice name for a set?"
• "Why is y? A ha ha ha."
• "More exciting sets."
• "Power sets are funny."
• "If you do not have enough power in your sets you need power sets."
• "This is a completely hunky-dory set."

• "I feed something into the children and nothing comes out."
• "It's just a machine. You put someone in and out the other side comes the mother. In nature it's the other way around."
• "Hello. Hello. Hello. Hello."
• "It's a magic trick."
• "If you want to pronounce his name in a funny way then I don't mind."
• "If the sun shines yet we still do not go to the zoo, then my children will be crying, yah?"
• "You write on your paper."
• "It's always the same dolly, you can do it forever, yah?"
• "... but that is trivial becuase 'less than equal' looks very boring."
• "Here's a nice square universe."
• "That's why you're at university. To learn about the universe."
• "You have to fix the universe."
• "I should have a barber, I almost cut myself."
• "Has anybody got a mirror?"

• "Do you really understand what I write there?"
• "Did I say upper? I mean lower."
• "Up and down, left and right - I'm not very good."
• "Not a football field, just a field"
• "Points on a plane. Not an aeroplane. A plane like an infinitely big piece of paper."
• "Huh? Huh? What's going on here?"
• "That's not what I wanted to do!"
• "Now I have confused myself!"

• "Who thinks she is right?... Who thinks she is wrong?... Who thinks nothing?..."
• "It is not cool to put your hands up, yah?"
• "Maybe you like to put both hands up, yah?"
• "Hands up, who does not want to raise his arm?"
• "Nobody will sit here anymore. You will all sit in the last row."
• "Who thinks it's not funny?"
• "Nobody thinks anything here!"
• "Sometimes nobody answers because it is just too stupid."

• "If you don't understand this now you might understand it next year, so don't worry."
• "If enlightenment doesn't come immediately you need practice. It's like Buddhism."
• "If there are 3 people and they each give me one third of a cake, how many cakes do I have?"
• "What's three times two? Ummm..... Ummm..... six!"
• "Where do I get this idea? It doesn't just fall from the sky you know!"
• "My answer sheet is mostly right."
• "I only want to demonstrate that I am not perfect either."
• "I have to apologise that I am not perfect."
• "I did something bad."
• "I get 3 points because I got 3 right and 2 wrong. That's over 50%!"
• "I was never very good at arithmetic so I get no points!"
• "For every point you get right you get one point.... that's how I calculate it."
• "If you find an error on the slide then there are 2 possibilities. Either you are too stupid to realise what's going on, or I have made a mistake."
• "Everybody drinks in an empty pub."
• "It's a good way to earn money - with logic."
• "Sometimes common sense doesn't work."
• "If you still don't believe me, you can bet ten pounds."
• "I obviously messed up this trivial example... Try doing it yourself!"
• "Think about it in the bath tub."

• "It's a green pen, ooo!"
• "Can anybody read the blue pen, actually?"
• "The green pen - ugh, I used the green pen."
• "This black pen may be readable but it makes a horrible noise."

• "I think I'm going to talk about lettuces.... no, lattices.... how do you spell that?"
• "Do you use the word lettuce in everyday life or is it a funny word?"
• student: "It's lattice" Thorsten: "With an 'a' - not with a 'u'?"
• "Lettuce, lattice, I dunno."
• "It's not a lattice. It looks more like a collar."

• [mobile phone beeps] Thorsten:  "Beep!"
• [mobile phone rings] Thorsten: "Hello?"
• [mobile phone rings] Thorsten: "Your phone is ringing."

• "You are falling asleep?"
• "A party in your corridor? It's a shame - life can be tough."
• "You've all been to a party?"

• "Here's a funny function."
• "Infinite sets are getting a bit funny."
• "This operation is a funny one."
• "I use some funny piece of syntax."

Miscellaneous

• "Shorter is a good word, yah?"
• "I am there lonely in my office. Nobody comes to see me."
• "I try to put a bit of German in everything."
• [to student] "Who's your tutor?...... Hmmm, a Chinese name...... What's the surname again?...... Hmmm......   ...... Ok...... Let's assume I'm your tutor."
• "In English 'rest' means to sleep ... In German, 'rest' means what's left over."
• "Today we're going to talk about pubs."
• "We will not talk about the football, yah?"  [after Germany's 1-0 victory over England]
• "People may be of different opinions whether I am funny or not." [Thorsten is still modest about his comic genius]